Work vehicle and control method for same

ABSTRACT

The power-transmission device has an input shaft, an output shaft, a gear mechanism, and a motor. The gear mechanism includes a plurality of planetary gear mechanisms and a mode-switching mechanism, and transmits the rotations of the input shaft to the output shaft. The mode-switching mechanism selectively switches the drive-power transmission path of the power-transmission device between a plurality of modes. The motor is connected to the rotating elements of the planetary gear mechanisms. A target-input-torque determination unit determines the target input torque, which is a target value for the torque to be inputted to the power-transmission device. The target-output-torque determination unit determines the target output torque, which is a target value for the torque to be outputted from the power-transmission device. The command-torque determination unit uses the torque balance information to determine torque commands to the motor from the target input torque and the target output torque.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National stage application of InternationalApplication No. PCT/JP2014/066881, filed on Jun. 25, 2014. This U.S.National stage application claims priority under 35 U.S.C. §119(a) toJapanese Patent Application No. 2013-136243, filed in Japan on Jun. 28,2013, the entire contents of which are hereby incorporated herein byreference.

BACKGROUND

Field of the Invention

The present invention relates to a work vehicle and a control methodtherefor.

Background Information

A power-transmission device (referred to hereinbelow as a “torqueconverter-type transmission”) having a torque converter and amulti-stage speed change gear is well known as a work vehicle, such as awheel loader. However, recently hydraulic-mechanical transmissions (HMT)have become known as power-transmission devices in place of torqueconverter-type transmissions. As disclosed in Japanese Laid-Open PatentPublication No. 2006-329244, an HMT has a gear mechanism and a motorconnected to rotation elements of the gear mechanism, and a portion ofthe driving power from the engine is converted to hydraulic pressure andtransmitted to a travel device, and the remaining portion of the drivingpower is mechanically transmitted to the travel device.

The HMT is provided with a planetary gear mechanism and a hydraulicmotor, for example, in order to allow stepless speed variation. A firstelement among the three elements of a sun gear, a carrier, and a ringgear of the planetary gear mechanism is coupled to an input shaft, andthe second element is coupled to an output shaft. The third element iscoupled to the hydraulic motor. The hydraulic motor functions as eithera motor or a pump in response to the travel state of the work vehicle.The HMT is configured to enable stepless changing of the rotation speedof the output shaft by changing the rotation speed of the hydraulicmotor.

Further as described in Japanese Laid-Open Patent Publication No.2008-247269, an electric-mechanical transmission device (EMT) has beenproposed as a technique similar to the HMT. An electric motor is used inthe EMT in place of the hydraulic motor in the HMT. The electric motorfunctions as either a motor or a generator in response to the travelstate of the work vehicle. Similar to the HMT, the EMT is configured toenable stepless changing of the rotation speed of the output shaft bychanging the rotation speed of the electric motor.

SUMMARY

The vehicle speed and the tractive force follow predetermined tractiveforce characteristics in a work vehicle provided with the conventionaltorque converter-type transmission. The tractive force characteristicsare determined by the torque converter characteristics and the speedchange ratio of the transmission, and are designed to be characteristicssuited to the vehicle.

Conversely, the HMT and the EMT do not necessarily exhibit predeterminedtractive force characteristics, such as the torque converter. However,it is important that the predetermined tractive force characteristicsare accurate to allow the operator to operate the work vehicle in astable manner.

Furthermore, while the predetermined tractive force characteristics canbe obtained in the conventional torque converter-type transmission, itis not easy to change the tractive force characteristics to set thedesired tractive force characteristics. That is, because the range ofthe tractive force characteristics that can be set is constrained by themechanical structure of the torque converter, it is difficult to changethe tractive force characteristics.

An object of the present invention is to provide a work vehicle and acontrol method for a work vehicle that enable a high degree of freedomfor setting the tractive force characteristics and allow predeterminedtractive force characteristics to be obtained accurately.

A work vehicle according to a first exemplary embodiment of the presentinvention is equipped with an engine, a hydraulic pump, a workimplement, a travel device, a power-transmission device, and a controlunit. The hydraulic pump is driven by the engine. The work implement isdriven by hydraulic fluid discharged from the hydraulic pump. The traveldevice is driven by the engine. The power-transmission device transmitsdriving power from the engine to the travel device. The control unitcontrols the power-transmission device. The power-transmission devicehas an input shaft, an output shaft, a gear mechanism, and a motor. Thegear mechanism includes a plurality of planetary gear mechanisms and amode-switching mechanism, and transmits the rotations of the input shaftto the output shaft. The mode-switching mechanism selectively switchesthe drive-power transmission path in the power-transmission device amonga plurality of modes. The motor is connected to a rotating element ofthe planetary gear mechanisms. The power-transmission device isconfigured so that a rotation speed ratio of the output shaft withrespect to the input shaft is changed by changing the rotation speed ofthe motor.

The control unit has a target-input-torque determination unit, atarget-output-torque determination unit, a storage unit, and acommand-torque determination unit. The target-input-torque determinationunit determines the target input torque. The target input torque is atarget value for the torque to be inputted to the power-transmissiondevice. The target-output-torque determination unit determines thetarget output torque. The target output torque is a target value for thetorque to be outputted from the power-transmission device. The storageunit stores torque balance information. The torque balance informationprescribes a relationship between the target input torque and the targetoutput torque to achieve a balance of the torques of thepower-transmission device. The command-torque determination unit usesthe torque balance information to determine a command torque to themotor from the target input torque and the target output torque.

A desired input torque to the power-transmission device and a desiredoutput torque from the power-transmission device can be achieved in thework vehicle by determining the command torque to the motor from thebalance of the torques of the power-transmission device. As a result,predetermined tractive force characteristics can be achieved accurately.The tractive force characteristics can be changed easily by changing thetarget input torque and the target output torque. As a result, a highdegree of freedom for setting the tractive force characteristics isachieved.

The work vehicle is preferably further provided with a vehicle speeddetecting unit, an accelerator operating member, and an acceleratoroperation detecting unit. The vehicle speed detecting unit detects thevehicle speed. The accelerator operation detecting unit detects anoperation amount of the accelerator operating member. The control unitfurther has a transmission requirement determination unit. Thetransmission requirement determination unit determines a requiredtractive force on the basis of the vehicle speed and the operationamount of the accelerator operating member. The target-output-torquedetermination unit determines the target output torque on the basis ofthe required tractive force.

In this case, the required tractive force is determined on the basis ofthe operation amount of the accelerator operating member in addition tothe vehicle speed. That is, the target output torque is determined inresponse to the operation of the accelerator operating member by theoperator. As a result, the operational feeling of the operator can beimproved.

The transmission requirement determination unit preferably determinesthe required tractive force from the vehicle speed on the basis of therequired tractive force characteristics. The required tractive forcecharacteristics define a relationship between the vehicle speed and therequired tractive force. The transmission requirement determination unitdetermines the required tractive force characteristics on the basis ofthe operation amount of the accelerator operating member. In this case,the operational feeling of the operator can be improved because therequired tractive force characteristics are determined in response tothe operation of the accelerator operating member by the operator.

The transmission requirement determination unit preferably determinesthe required tractive force characteristics by multiplying the basicrequired tractive force characteristics by a tractive force ratio and avehicle speed ratio. The transmission requirement determination unitdetermines the tractive force ratio and the vehicle speed ratio on thebasis of the operation amount of the accelerator operating member. Inthis case, the required tractive force characteristics can be determinedin response to the operation amount of the accelerator operating memberby using the tractive force ratio and the vehicle speed ratio inresponse to the operation amount of the accelerator operating member.

The work vehicle is preferably further provided with a speed changeoperating member. The transmission requirement determination unitselects the above basic required tractive force characteristics inresponse to the operation of the speed change operating member. In thiscase, the desired tractive force characteristics can be selected fromthe operation of the speed change operating member.

The required tractive force characteristics preferably define a requiredtractive force that is a negative value with respect to a vehicle speedthat is equal to or greater than a predetermined speed. In this case,the required tractive force becomes a negative value when the vehiclespeed is equal to or greater than the predetermined speed. That is, whenthe vehicle speed is high, the power-transmission device is controlledto generate a braking force.

The work vehicle is preferably further provided with an energy reservoirunit. The energy reservoir unit stores energy generated in the motor.The control unit further has an energy management requirementdetermination unit. The energy management requirement determination unitdetermines an energy management required horsepower on the basis of aremaining amount of energy in the energy reservoir unit. Thetransmission requirement determination unit determines a transmissionrequired horsepower on the basis of the vehicle speed and the operationamount of the accelerator operating member. The target-input-torquedetermination unit determines the target input torque on the basis ofthe transmission required horsepower and the energy management requiredhorsepower.

In this case, the target input torque can be determined to obtain thetransmission required horsepower required for outputting a tractiveforce corresponding to the required tractive force from thepower-transmission device, and the energy management required horsepowerrequired for storing energy in the energy reservoir unit.

The work vehicle is preferably further provided with a work implementoperating member for operating the work implement. The control unitfurther has a work implement requirement determination unit and anengine requirement determination unit. The work implement requirementdetermination unit determines a work implement required horsepower onthe basis of the operation amount of the work implement operatingmember. The engine requirement determination unit determines an enginerequired horsepower on the basis of the work implement requiredhorsepower, the transmission required horsepower, and the energymanagement required horsepower. The target-input-torque determinationunit determines an upper limit of the target input torque from an uppertarget input torque line and the engine rotation speed. The upper targetinput torque line defines, as the upper limit of the target inputtorque, a value that is less than the target output torque of the enginedetermined from the engine required horsepower and the engine rotationspeed.

In this case, a value less than the target output torque of the enginedetermined from the engine required horsepower and the engine rotationspeed becomes the upper limit of the target input torque. As a result,the target input torque to the power-transmission device is determinedso that excess torque for increasing the engine rotation speed remains.As a result, a decrease in the engine rotation speed due to overloadingcan be suppressed.

The control unit preferably further has a distribution ratiodetermination unit. The distribution ratio determination unit determinesa transmission output ratio. The distribution ratio determination unitsets a value less than one as the transmission output ratio when thetotal of the work implement required horsepower, the transmissionrequired horsepower, and the energy management required horsepower islarger than a predetermined load upper limit horsepower. Thetarget-input-torque determination unit determines the target inputtorque on the basis of the energy management required horsepower and avalue derived by multiplying the transmission required horsepower by thetransmission output ratio.

In this case, while the value of the transmission required horsepower isreduced when determining the target input torque when the total of thework implement required horsepower, the transmission requiredhorsepower, and the energy management required horsepower is larger thanthe predetermined load upper limit horsepower, the value of the energymanagement required horsepower is maintained. That is, the energymanagement required horsepower is prioritized more than the transmissionrequired horsepower when determining the target input torque.Consequently, the energy management required horsepower is prioritizedand the output horsepower of the engine can be distributed, and as aresult a predetermined amount of energy can be ensured in the energyreservoir unit.

The target-output-torque determination unit preferably determines thetarget output torque on the basis of a value derived by multiplying therequired tractive force by the transmission output ratio. In this case,the energy management required horsepower is prioritized more than therequired tractive force when determining the target output torque. As aresult, a predetermined amount of energy can be ensured in the energyreservoir unit.

The control unit preferably further has the engine requirementdetermination unit and a required throttle determination unit. Theengine requirement determination unit determines the engine requiredhorsepower. The required throttle determination unit determines arequired throttle value. The storage unit stores an engine torque lineand a matching line. The engine torque line defines a relationshipbetween the engine output torque and the engine rotation speed. Thematching line is information for determining the required throttle valuefrom the engine required horsepower.

The engine torque line includes a regulation region and a full loadregion. The regulation region changes in response to the requiredthrottle value. The full load region includes a rated point and amaximum torque point located on the low engine rotation speed side fromthe rated point. The required throttle determination unit determines therequired throttle value so that the engine torque line and the matchingline coincide at a matching point where the output torque of the enginebecomes the torque corresponding to the engine required horsepower. Thematching line is set to pass through a position closer to the maximumtorque point than the rated point in the full load region of the enginetorque line.

In this case, the engine rotation speed at the matching point is reducedin comparison to when the matching line is set to pass through alocation closer to the rated point than the maximum torque point in thefull load region. As a result, the fuel consumption can be improved.

The work vehicle is preferably further provided with the work implementoperating member, the vehicle speed detecting unit, the acceleratoroperating member, the accelerator operation detecting unit, and theenergy reservoir unit. The work implement operating member is a memberfor operating the work implement. The vehicle speed detecting unitdetects the vehicle speed. The accelerator operation detecting unitdetects an operation amount of the accelerator operating member. Theenergy reservoir unit stores energy generated in the motor. The controlunit further has the work implement requirement determination unit, thetransmission requirement determination unit, and the energy managementrequirement determination unit.

The work implement requirement determination unit determines the workimplement required horsepower on the basis of the operation amount ofthe work implement operating member. The transmission requirementdetermination unit determines the transmission required horsepower onthe basis of the vehicle speed and the operation amount of theaccelerator operating member. The energy management requirementdetermination unit determines the energy management required horsepoweron the basis of a remaining amount of energy in the energy reservoirunit. The engine requirement determination unit determines the enginerequired horsepower on the basis of the work implement requiredhorsepower, the transmission required horsepower, and the energymanagement required horsepower.

In this case, the engine required horsepower is determined that issuited for the driving of the work implement and the driving of thetravel device in response to the operation each of the operating membersby the operator, and for the storage of energy in the energy reservoirunit.

A plurality of modes preferably includes a first mode and a second mode.The command-torque determination unit determines a command torque to themotor from a first torque balance information in the first mode. Thecommand-torque determination unit determines a command torque to themotor from a second torque balance information in the second mode. Inthis case, the command-torque determination unit is able to determine acommand torque suited for the selected mode.

A control method according to a second exemplary embodiment of thepresent invention is a control method for a work vehicle. The workvehicle is provided with a power-transmission device. Thepower-transmission device has an input shaft, an output shaft, a gearmechanism, and a motor. The gear mechanism includes a plurality ofplanetary gear mechanisms and a mode-switching mechanism, and transmitsthe rotations of the input shaft to the output shaft. The mode-switchingmechanism selectively switches the drive-power transmission path of thepower-transmission device among a plurality of modes. The motor isconnected to a rotating element of the planetary gear mechanisms. Thepower-transmission device is configured to change the rotation speedratio of the output shaft with respect to the input shaft by changingthe rotation speed of the motor. The control method according to thepresent aspect includes the following steps. A target input torque thatis a torque target value inputted to the power-transmission device isdetermined in a first step. A target output torque that is a torquetarget value outputted from the power-transmission device is determinedin a second step. A command torque for the motor is determined on thebasis of the target input torque and the target output torque by usingtorque balance information that defines a relationship between thetarget input torque and the target output torque so that a balancebetween the toques is achieved in the power-transmission device in athird step.

A desired input torque for the power-transmission device and a desiredoutput torque for the power-transmission device can be achieved in thecontrol method of the work vehicle by determining the command torque tothe motor from the balance of the torques of the power-transmissiondevice. As a result, predetermined tractive force characteristics can beprovided accurately. The tractive force characteristics can be changedeasily by changing the target input torque and the target output torque.As a result, a high degree of freedom for setting the tractive forcecharacteristics is achieved.

An object of the present invention is to provide a work vehicle and acontrol method for the work vehicle that enable a high degree of freedomfor setting the tractive force characteristics and allow predeterminedtractive force characteristics to be obtained accurately.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a work vehicle according to an exemplaryembodiment of the present invention.

FIG. 2 is a schematic view of a configuration of the work vehicle.

FIG. 3 is a schematic view of a configuration of a power-transmissiondevice.

FIG. 4 illustrates changes in rotation speeds of a first motor and asecond motor with respect to the vehicle speed.

FIG. 5 is a control block diagram illustrating an overall outline ofprocessing executed by a control unit.

FIG. 6 is a control block diagram illustrating processing executed bythe control unit to determine command torques.

FIG. 7 is a control block diagram illustrating processing executed bythe control unit to determine a target output torque.

FIG. 8 is a control block diagram illustrating processing executed bythe control unit to determine transmission required horsepower andrequired tractive force.

FIG. 9 is a control block diagram illustrating processing executed bythe control unit to determine a target input torque.

FIG. 10 is a control block diagram illustrating processing executed bythe control unit to determine an upper limit target input torque.

FIG. 11 is a control block diagram illustrating processing executed bythe control unit to determine a work implement output flow rate, a workimplement required horsepower and a work implement load torque.

FIG. 12 is a control block diagram illustrating processing executed bythe control unit to determine an engine required horsepower, atransmission output ratio and a work implement output ratio.

FIG. 13 is a control block diagram illustrating processing executed bythe control unit to determine a command throttle value.

FIG. 14 illustrates a method for distributing output horsepower from anengine.

FIG. 15 illustrates operations of the work vehicle while carrying out aV-shaped work.

FIGS. 16A-16H are timing charts illustrating changes in parameters ofthe work vehicle carrying out a V-shaped work.

FIG. 17 is a schematic view of a configuration of a power-transmissiondevice according to another exemplary embodiment.

FIG. 18 illustrates changes in the rotation speeds of the first motorand the second motor with respect to the vehicle speed in thepower-transmission device according to still another exemplaryembodiment.

FIG. 19 illustrates a method for distributing output horsepower from anengine according to yet another exemplary embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the following description, exemplary embodiments of the presentinvention will be described with reference to the accompanying drawings.FIG. 1 is a side view of a work vehicle 1 according to an exemplaryembodiment of the present invention. As illustrated in FIG. 1, the workvehicle 1 is equipped with a vehicle body frame 2, a work implement 3,traveling wheels 4 and 5, and an operating cabin 6. The work vehicle 1is a wheel loader and travels due to the traveling wheels 4 and 5 beingrotated and driven. The work vehicle 1 is able to carry out work, suchas excavation, by using the work implement 3.

The work implement 3 and the traveling wheels 4 are attached to thevehicle body frame 2. The work implement 3 is driven by hydraulic fluidfrom a below mentioned work implement pump 23 (see FIG. 2). The workimplement 3 has a boom 11 and a bucket 12. The boom 11 is mounted on thevehicle body frame 2. The work implement 3 includes a lift cylinder 13and a bucket cylinder 14. The lift cylinder 13 and the bucket cylinder14 are hydraulic cylinders. One end of the lift cylinder 13 is attachedto the vehicle body frame 2. The other end of the lift cylinder 13 isattached to the boom 11. The boom 11 swings up and down due to theextension and contraction of the lift cylinder 13 due to hydraulic fluidfrom the work implement pump 23. The bucket 12 is attached to the tip ofthe boom 11. One end of the bucket cylinder 14 is attached to thevehicle body frame 2. The other end of the bucket cylinder 14 isattached to the bucket 12 via a bell crank 15. The bucket 12 swings upand down due to the extension and contraction of the bucket cylinder 14due to hydraulic fluid from the work implement pump 23.

The operating cabin 6 and the traveling wheels 5 are attached to thevehicle body frame 2. The operating cabin 6 is mounted on the vehiclebody frame 2. A seat for the operator and a below mentioned operatingdevice are disposed in the operating cabin 6. The vehicle body frame 2has a front frame 16 and a rear frame 17. The front frame 16 and therear frame 17 are attached to each other in a manner that allowsswinging in the left-right direction.

The work vehicle 1 has a steering cylinder 18. The steering cylinder 18is attached to the front frame 16 and the rear frame 17. The steeringcylinder 18 is a hydraulic cylinder. The work vehicle 1 is able tochange the advancing direction to the right and left with the extensionand contraction of the steering cylinder 18 due to hydraulic fluid froma below mentioned steering pump 30 (FIG. 2).

FIG. 2 is a schematic view of a configuration of the work vehicle 1. Asillustrated in FIG. 2, the work vehicle 1 is equipped with an engine 21,a PTO (power take-off) 22, a power-transmission device 24, a traveldevice 25, an operating device 26, and a control unit 27.

The engine 21 is, for example, a diesel engine. The output of the engine21 is controlled by adjusting the amount of fuel injected into thecylinders of the engine 21. The adjustment of the amount of fuel isconducted by the control unit 27 controlling a fuel injection device 28attached to the engine 21. The work vehicle 1 is equipped with an enginerotation speed detecting unit 31. The engine rotation speed detectingunit 31 detects the engine rotation speed and transmits a detectionsignal indicating the engine rotation speed to the control unit 27.

The work vehicle 1 has the work implement pump 23, the steering pump 30,and a transmission pump 29. The work implement pump 23, the steeringpump 30, and the transmission pump 29 are hydraulic pumps. The PTO 22(power take-off) transmits a portion of the driving power from theengine 21 to the hydraulic pumps 23, 30, and 29. That is, the PTO 22distributes the driving power from the engine 21 to thepower-transmission device 24 and to the hydraulic pumps 23, 30, and 29.

The work implement pump 23 is driven by driving power from the engine21. Hydraulic fluid discharged from the work implement pump 23 issupplied to the lift cylinder 13 and the bucket cylinder 14abovementioned through a work implement control valve 41. The workimplement control valve 41 changes the flow rate of the hydraulic fluidsupplied to the lift cylinder 13 and to the bucket cylinder 14 inresponse to an operation of a below mentioned work implement operatingmember 52 a. The work vehicle 1 is equipped with a work implement pumppressure detecting unit 32. The work implement pump pressure detectingunit 32 detects a discharge pressure (referred to below as “workimplement pump pressure”) of hydraulic fluid from the work implementpump 23 and transmits a detection signal indicating the work implementpump pressure to the control unit 27.

The work implement pump 23 is a variable displacement hydraulic pump.The discharge capacity of the work implement pump 23 is changed bychanging the tilt angle of a skew plate or an inclined shaft of the workimplement pump 23. A first capacity control device 42 is connected tothe work implement pump 23. The first capacity control device 42 iscontrolled by the control unit 27 and changes the tilt angle of the workimplement pump 23. As a result, the discharge capacity of the workimplement pump 23 is controlled by the control unit 27. For example, thefirst capacity control device 42 adjusts the tilt angle of the workimplement pump 23 so that a differential pressure between the front andthe rear of the work implement control valve 41 is fixed. The firstcapacity control device 42 also optionally changes the tilt angle of thework implement pump 23 in response to a command signal from the controlunit 27. Specifically, the first capacity control device 42 includes afirst valve and a second valve (not illustrated). When the hydraulicfluid supplied to the work implement 3 is changed due to theabovementioned work implement control valve 41, a differential pressureis generated between the discharge pressure of the work implement pump23 and the pressure after passing through the work implement controlvalve 41 in response to the change in the opening degree of the workimplement control valve 41. The first valve is controlled by the controlunit 27 to adjust the tilt angle of the work implement pump 23 so thatthe differential pressure between the front and rear of the workimplement control valve 41 is fixed even if the load on the workimplement 3 fluctuates. The second valve is controlled by the controlunit 27 to be able to further change the tilt angle of the workimplement pump 23. The work vehicle 1 is equipped with a first tiltangle detecting part 33. The first tilt angle detecting part 33 detectsthe tilt angle of the work implement pump 23 and transmits a detectionsignal indicating the tilt angle to the control unit 27.

The steering pump 30 is driven by driving power from the engine 21. Thehydraulic fluid discharged from the steering pump 30 is supplied to theabove mentioned steering cylinder 18 through a steering control valve43. The work vehicle 1 is equipped with a steering pump pressuredetecting unit 34. The steering pump pressure detecting unit 34 detectsthe discharge pressure (referred to below as “steering pump pressure”)of hydraulic fluid from the steering pump 30 and transmits a detectionsignal indicating the steering pump pressure to the control unit 27.

The steering pump 30 is a variable displacement hydraulic pump. Thedischarge capacity of the steering pump 30 is changed by changing thetilt angle of a skew plate or an inclined shaft of the steering pump 30.A second capacity control device 44 is connected to the steering pump30. The second capacity control device 44 is controlled by the controlunit 27 and changes the tilt angle of the steering pump 30. As a result,the discharge capacity of the steering pump 30 is controlled by thecontrol unit 27. The work vehicle 1 is equipped with a second tilt angledetecting part 35. The second tilt angle detecting part 35 detects thetilt angle of the steering pump 30 and transmits a detection signalindicating the tilt angle to the control unit 27.

The transmission pump 29 is driven by driving power from the engine 21.The transmission pump 29 is a fixed displacement hydraulic pump.Hydraulic fluid discharged from the transmission pump 29 is supplied toclutches CF, CR, CL, and CH of the power-transmission device 24 viabelow mentioned clutch control valves VF, VR, VL, and VH. The workvehicle 1 is equipped with a transmission pump pressure detecting unit36. The transmission pump pressure detecting unit 36 detects thedischarge pressure (referred to below as “transmission pump pressure”)of the hydraulic fluid from the transmission pump 29 and transmits adetection signal indicating the transmission pump pressure to thecontrol unit 27.

The PTO 22 transmits a portion of the driving power from the engine 21to the power-transmission device 24. The power-transmission device 24transmits the driving power from the engine 21 to the travel device 25.The power-transmission device 24 changes the speed and outputs thedriving power from the engine 21. An explanation of the configuration ofthe power-transmission device 24 is provided in detail below.

The travel device 25 has an axle 45 and the traveling wheels 4 and 5.The axle 45 transmits driving power from the power-transmission device24 to the traveling wheels 4 and 5. As a result, the traveling wheels 4and 5 rotate. The work vehicle 1 is equipped with a vehicle speeddetecting unit 37. The vehicle speed detecting unit 37 detects therotation speed (referred to below as “output rotation speed”) of anoutput shaft 63 of the power-transmission device 24. The output rotationspeed corresponds to the vehicle speed and consequently the vehiclespeed detecting unit 37 detects the vehicle speed by detecting theoutput rotation speed. The vehicle speed detecting unit 37 detects therotating direction of the output shaft 63. The rotating direction of theoutput shaft 63 corresponds to the traveling direction of the workvehicle 1 and consequently the vehicle speed detecting unit 37 detectsthe traveling direction of the work vehicle 1 by detecting the rotatingdirection of the output shaft 63. The vehicle speed detecting unit 37transmits detection signals indicating the output rotation speed and therotating direction to the control unit 27.

The operating device 26 is operated by the operator. The operatingdevice 26 has an accelerator operating device 51, a work implementoperating device 52, a speed change operating device 53, a FR operatingdevice 54, a steering operating device 57, and a brake operating device58.

The accelerator operating device 51 has an accelerator operating member51 a and an accelerator operation detecting unit 51 b. The acceleratoroperating member 51 a is operated to set a target rotation speed of theengine 21. The accelerator operation detecting unit 51 b detects anoperation amount (referred to below as “accelerator operating amount”)of the accelerator operating member 51 a. The accelerator operationdetecting unit 51 b transmits a detection signal indicating theaccelerator operating amount to the control unit 27.

The work implement operating device 52 has a work implement operatingmember 52 a and a work implement operation detecting unit 52 b. The workimplement operating member 52 a is operated to actuate the workimplement 3. The work implement operation detecting unit 52 b detects aposition of the work implement operating member 52 a. The work implementoperation detecting unit 52 b outputs a detection signal indicating theposition of the work implement operating member 52 a to the control unit27. The work implement operation detecting unit 52 b detects anoperation amount (referred to below as “boom operation amount”) of thework implement operating member 52 a for operating the boom 11 and anoperation amount (referred to below as “bucket operation amount”) of thework implement operating member 52 a for operating the bucket 14, bydetecting the position of the work implement operating member 52 a. Thework implement operating member 52 is configured for example with onelever and the operation of the boom 11 and the operation of the bucket14 may be assigned to each operating direction of the lever.Alternatively, the work implement operating member 52 is configured, forexample, with two levers and the operation of the boom 11 and theoperation of the bucket 14 may be assigned to each lever.

The speed change operating device 53 has a speed change operating member53 a and a speed change operation detecting unit 53 b. The operator isable to select a speed range of the power-transmission device 24 byoperating the speed change operating member 53 a. The speed changeoperation detecting unit 53 b detects a position of the speed changeoperating member 53 a. The position of the speed change operating member53 a corresponds to a plurality of speed ranges such as a first speedand a second speed and the like. The speed change operation detectingunit 53 b outputs a detection signal indicating the position of thespeed change operating member 53 a to the control unit 27.

The FR operating device 54 has a FR operating member 54 a and a FRoperation detecting unit 54 b. The operator can switch between forwardand reverse movement of the work vehicle 1 by operating the FR operatingmember 54 a. The FR operating member 54 a is selectively switchedbetween a forward movement position (F), a neutral position (N), and areverse movement position (R). The FR operation detecting unit 54 bdetects the position of the FR operating member 54 a. The FR operationdetecting unit 54 b outputs a detection signal indicating the positionof the FR operating member 54 a to the control unit 27.

The steering operating device 57 has a steering operating member 57 aand a steering operation detecting unit 57 b. The operator is able tochange the travel direction of the work vehicle 1 to the right or leftby operating the steering operating member 57 a. The steering operationdetecting unit 57 b detects a position of the steering operating member57 a. The steering operation detecting unit 57 b outputs a detectionsignal indicating the position of the steering operating member 57 a tothe control unit 27.

The brake operating device 58 has a brake operating member 58 a and abrake operation detecting unit 58 b. The operator actuates a brakedevice (not illustrated) to generate a braking force on the work vehicle1 by operating the brake operating member 58 a. The brake operationdetecting unit 58 b detects the position of the brake operating member58 a. The brake operation detecting unit 58 b outputs a detection signalindicating the position of the brake operating member 58 a to thecontrol unit 27.

The control unit 27 has a calculation device, such as a CPU, and amemory, such as a RAM or a ROM, and conducts various types of processingfor controlling the work vehicle 1. The control unit 27 has a storageunit 56. The storage unit 56 stores various types of programs and datafor controlling the work vehicle 1.

The control unit 27 transmits a command signal indicating a commandthrottle value to the fuel injection device 28 to achieve the targetrotation speed of the engine 21 in response to the accelerator operatingamount. The control of the engine 21 by the control unit 27 is describedin detail below.

The control unit 27 controls hydraulic pressure of the hydraulic fluidto be supplied to the hydraulic cylinders 13 and 14 by controlling thework implement control valve 41 on the basis of the detection signalsfrom the work implement operation detecting unit 52 b. As a result, thehydraulic cylinders 13 and 14 expand or contract to operate the workimplement 3.

The control unit 27 controls the hydraulic pressure of the hydraulicfluid to be supplied to the steering cylinder 18 by controlling thesteering control valve 43 on the basis of the detection signals from thesteering operation detecting unit 57 b. As a result, the steeringcylinder 18 is extended and contracted and the traveling direction ofthe wheel loader 1 is changed.

The control unit 27 controls the power-transmission device 24 on thebasis of the detection signals from each of the detecting units. Thecontrol of the power-transmission device 24 by the control unit 27 isdescribed in detail below.

An explanation of the configuration of the power-transmission device 24is provided in detail below. FIG. 3 is a schematic view of aconfiguration of the power-transmission device 24. As illustrated inFIG. 3, the power-transmission device 24 is provided with an input shaft61, a gear mechanism 62, the output shaft 63, a first motor MG1, asecond motor MG2, and a capacitor 64. The input shaft 61 is connected tothe abovementioned PTO 22. The rotation from the engine 21 is inputtedto the input shaft 61 via the PTO 22. The gear mechanism 62 transmitsthe rotation of the input shaft 61 to the output shaft 63. The outputshaft 63 is connected to the abovementioned travel device 25, andtransmits the rotation from the gear mechanism 62 to the above mentionedtravel device 25.

The gear mechanism 62 is a mechanism for transmitting driving power fromthe engine 21. The gear mechanism 62 is configured so that the rotationspeed ratio of the output shaft 63 with respect to the input shaft 61 ischanged in response to changes in the rotation speeds of the motors MG1and MG2. The gear mechanism 62 has a FR switch mechanism 65, and a speedchange mechanism 66.

The FR switch mechanism 65 selectively switches a drive-powertransmission path of the power-transmission device 24 between a forwardmovement travel mode and a reverse movement travel mode. The FR switchmechanism 65 has various types of gears (not illustrated) such as aforward movement clutch CF and a reverse movement clutch CR. The forwardmovement clutch CF and the reverse movement clutch CR are hydraulicclutches and hydraulic fluid is supplied from the transmission pump 29to each of the clutches CF and CR. The hydraulic fluid for the forwardmovement clutch CF is controlled by an F-clutch control valve VF. Thehydraulic fluid for the reverse movement clutch CR is controlled by anR-clutch control valve VR. Each of the clutch control valves CF and CRare controlled by command signals from the control unit 27. Thedirection of the rotation outputted from the FR switch mechanism 65 isswitched due to the switching between ON (connection)/OFF(disconnection) of the forward movement clutch CF and ON(connection)/OFF (disconnection) of the reverse movement clutch CR.

The speed change mechanism 66 has a transmission shaft 67, a firstplanetary gear mechanism 68, a second planetary gear mechanism 69, aHi/Lo switch mechanism 70, and an output gear 71. The transmission shaft67 is coupled to the FR switch mechanism 65. The first planetary gearmechanism 68 and the second planetary gear mechanism 69 are disposed onthe same shaft as the transmission shaft 67.

The first planetary gear mechanism 68 has a first sun gear S1, aplurality of first planet gears P1, a first carrier C1 that supports theplurality of first planet gears P1, and a first ring gear R1. The firstsun gear S1 is coupled to the transmission shaft 67. The plurality offirst planet gears P1 mesh with the first sun gear S1 and are supportedin a rotatable manner by the first carrier C1. A first carrier gear Gc1is provided on an outer peripheral part of the first carrier C1. Thefirst ring gear R1 meshes with the plurality of first planet gears P1and is able to rotate. A first ring outer periphery gear Gr1 is providedon the outer periphery of the first ring gear R1.

The second planetary gear mechanism 69 has a second sun gear S2, aplurality of second planet gears P2, a second carrier C2 that supportsthe plurality of second planet gears P2, and a second ring gear R2. Thesecond sun gear S2 is coupled to the first carrier C1. The plurality ofsecond planet gears P2 mesh with the second sun gear S2 and aresupported in a rotatable manner by the second carrier C2. The secondring gear R2 meshes with the plurality of second planet gears P2 and isable to rotate. A second ring outer periphery gear Gr2 is provided onthe outer periphery of the second ring gear R2. The second ring outerperiphery gear Gr2 meshes with the output gear 71, and the rotation ofthe second ring gear R2 is outputted to the output shaft 63 via theoutput gear 71.

The Hi/Lo switch mechanism 70 is a mechanism for switching thedrive-power transmission path of the power-transmission device 24between a high-speed mode (Hi mode) in which the vehicle speed is highand a low-speed mode (Lo mode) in which the vehicle speed is low. TheHi/Lo switch mechanism 70 has an H-clutch CH that is ON during the Himode and an L-clutch CL that is ON during the Lo mode. The H-clutch CHconnects or disconnects the first ring gear R1 and the second carrierC2. The L-clutch CL connects or disconnects the second carrier C2 and afixed end 72 to prohibit or allow the rotation of the second carrier C2.

Each of the clutches CH and CL are hydraulic clutches, and hydraulicfluid from the transmission pump 29 is supplied to each of the clutchesCH and CL. The hydraulic fluid for the H-clutch CH is controlled by anH-clutch control valve VH. The hydraulic fluid for the L-clutch CL iscontrolled by an L-clutch control valve VL. Each of the clutch controlvalves VH and VL are controlled by command signals from the control unit27.

The first motor MG1 and the second motor MG2 function as drive motorsthat generate driving power using electrical energy. The first motor MG1and the second motor MG2 also function as generators that use inputteddriving power to generate electrical energy. The first motor MG1functions as a generator when a command signal from the control unit 27is applied to activate torque to the first motor MG1 in the reversedirection of the rotating direction of the first motor MG1. A firstmotor gear Gm1 is fixed to the output shaft of the first motor MG1 andthe first motor gear Gm1 meshes with the first carrier gear Gc1. A firstinverter I1 is connected to the first motor MG1 and a command signal forcontrolling the motor torque of the first motor MG1 is applied to thefirst inverter I1 from the control unit 27.

The second motor MG2 is configured in the same way as the first motorMG1. A second motor gear Gm2 is fixed to the output shaft of the secondmotor MG2 and the second motor gear Gm2 meshes with the first ring outerperiphery gear Gr1. A second inverter I2 is connected to the secondmotor MG2 and a command signal for controlling the motor torque of thesecond motor MG2 is applied to the second inverter I2 from the controlunit 27.

The capacitor 64 functions as an energy reservoir unit for storingenergy generated by the motors MG1 and MG2. That is, the capacitor 64stores electrical power generated by each of the motors MG1 and MG2 whenthe total electrical power generation amount of each of the motors MG1and MG2 is high. The capacitor 64 releases electrical power when thetotal electrical power consumption amount of each of the motors MG1 andMG2 is high. That is, each of the motors MG1 and MG2 are driven byelectrical power stored in the capacitor 64. A battery may be used asanother electrical power storage means in place of a capacitor.

The control unit 27 receives detection signals from the variousdetecting units and applies command signals for indicating the commandtorques for the motors MG1 and MG2 to each of the inverters I1 and I2.The control unit 27 also applies command signals for controlling theclutch hydraulic pressure of each of the clutches CF, CR, CH, and CL toeach of the clutch control valves VF, VR, VH, and VL. As a result, thespeed change ratio and the output torque of the power-transmissiondevice 24 are controlled. The following is an explanation of theoperations of the power-transmission device 24.

An outline of operations of the power-transmission device 24 when thevehicle speed increases from zero in the forward movement side while therotation speed of the engine 21 remains fixed, will be explained withreference to FIG. 4. FIG. 4 illustrates the rotation speeds of each ofthe motors MG1 and MG2 with respect to the vehicle speed. When therotation speed of the engine 21 is fixed, the vehicle speed changes inresponse to the rotation speed ratio of the power-transmission device24. The rotation speed ratio is the ratio of the rotation speed of theoutput shaft 63 with respect to the rotation speed of the input shaft61. Therefore, the variation in the vehicle speed in FIG. 4 matches thevariation of the rotation speed ratio of the power-transmission device24. That is, FIG. 4 illustrates the relationship between the rotationspeeds of each of the motors MG1 and MG2 and the rotation speed ratio ofthe power-transmission device 24. In FIG. 4, the solid line representsthe rotation speed of the first motor MG1, and the dashed linerepresents the rotation speed of the second motor MG2.

In an A region (Lo mode) with a vehicle speed from zero to V1, theL-clutch CL is ON (connected) and the H-clutch CH is OFF (disconnected).Because the H-clutch CH is OFF in the A region, the second carrier C2and the first ring gear R1 are disconnected. Because the L-clutch CL isON, the second carrier C2 is fixed.

In an A region, the driving power from the engine 21 is inputted to thefirst sun gear S1 via the transmission shaft 67, and the driving poweris outputted from the first carrier C1 to the second sun gear S2.Conversely, the driving power inputted to the first sun gear S1 istransmitted from the first planet gears P1 to the first ring gear R1 andoutputted through the first ring outer periphery gear Gr1 and the secondmotor gear Gm2 to the second motor MG2. In the A region, the secondmotor MG2 functions mainly as a generator, and a portion of theelectrical power generated by the second motor MG2 is stored in thecapacitor 64.

The first motor MG1 functions mainly as an electric motor in the Aregion. The driving power of the first motor MG1 is outputted to thesecond sun gear S2 along a path from the first motor gear Gm1 to thefirst carrier gear Gc1 to the first carrier C1. The driving poweroutputted to the second sun gear S2 as described above is transmitted tothe output shaft 63 along a path from the second planet gears P2 to thesecond ring gear R2 to the second ring outer periphery gear Gr2 to theoutput gear 71.

In a B region (Hi mode) in which the vehicle speed exceeds V1, theH-clutch CH is ON (connected) and the L-clutch CL is OFF (disconnected).Because the H-clutch CH is ON in the B region, the second carrier C2 andthe first ring gear R1 are connected. Because the L-clutch CL is OFF,the second carrier C2 is released. Therefore, the rotation speed of thefirst ring gear R1 and the second carrier C2 match.

In the B region, the driving power from the engine 21 is inputted to thefirst sun gear S1 and the driving power is outputted from the firstcarrier C1 to the second sun gear S2. The driving power inputted to thefirst sun gear S1 is outputted from the first carrier C1 through thefirst carrier gear Gc1 and the first motor gear Gm1 to the first motorMG1. In the B region, the first motor MG1 functions mainly as agenerator, and thus a portion of the electrical power generated by thefirst motor MG1 is stored in the capacitor 64.

The driving power of the second motor MG2 is outputted to the secondcarrier C2 along a path from the second motor gear Gm2 to the first ringouter periphery gear Gr1 to the first ring gear R1 to the H-clutch CH.The driving power outputted to the second sun gear S2 as described aboveis outputted through the second planet gears P2 to the second ring gearR2, and the driving power outputted to the second carrier C2 isoutputted through the second planet gears P2 to the second ring gear R2.The driving power joined by the second ring gear R2 in this way istransmitted through the second ring outer periphery gear Gr2 and theoutput gear 71 to the output shaft 63.

While forward movement driving has been discussed above, the operationsof reverse movement driving are the same. During braking, the roles ofthe first motor MG1 and the second motor MG2 as generator and motor arereversed from the above explanation.

The control of the power-transmission device 24 by the control unit 27is described in detail below. The control unit 27 controls the outputtorque of the power-transmission device 24 by controlling the motortorque of the first motor MG1 and the second motor MG2. That is, thecontrol unit 27 controls the tractive force of the work vehicle 1 bycontrolling the motor torque of the first motor MG1 and the second motorMG2.

A method for determining the command values (referred to below as“command torque”) of the motor torque to the first motor MG1 and thesecond motor MG2 is explained below. FIGS. 5 to 13 are control blockdiagrams illustrating processing executed by the control unit 27. Asillustrated in FIGS. 5 and 6, the control unit 27 has atarget-input-torque determination unit 81, a target-output-torquedetermination unit 82, and a command-torque determination unit 83.

The target-input-torque determination unit 81 determines a target inputtorque Te_ref. The target input torque Te_ref is a target value for thetorque to be inputted to the power-transmission device 24. Thetarget-output-torque determination unit 82 determines a target outputtorque To_ref. The target output torque To_ref is a target value for thetorque to be outputted from the power-transmission device 24. Thecommand-torque determination unit 83 uses torque balance information todetermine command torques Tm1_ref and Tm2_ref to the motors MG1 and MG2from the target input torque Te_ref and the target output torque To_ref.The torque balance information defines a relationship between the targetinput torque Te_ref and the target output torque To_ref so as to achievea balance of the torques of the power-transmission device 24. The torquebalance information is stored in the storage unit 56.

As described above, the transmission paths of the driving power in thepower-transmission device 24 are different for the Lo mode and the Himode. As a result, the command-torque determination unit 83 usesdifferent torque balance information to determine the command torquesTm1_ref and Tm2_ref for the motors MG1 and MG2 in the Lo mode and the Himode. Specifically, the command-torque determination unit 83 uses afirst torque balance information represented by equation 1 below todetermine command torques Tm1_Low and Tm2_Low for the motors MG1 and MG2in the Lo mode. In the present exemplary embodiment, the first torquebalance information is an equation for balancing the torques of thepower-transmission device 24.Ts1_Low=Te_ref*r_frTc1_Low=Ts1_Low*(−1)*((Zr1/Zs1)+1)Tr2_Low=To_ref*(Zod/Zo)Ts2_Low=Tr2_Low*(Zs2/Zr2)Tcp1_Low=Tc1_Low+Ts2_LowTm1_Low=Tcp1_Low*(−1)*(Zp1/Zp1d)Tr1_Low=Ts1_Low*(Zr1/Zs1)Tm2_Low=Tr1_Low*(−1)*(Zp2/Zp2d)  (Equation 1)

The command-torque determination unit 83 uses a second torque balanceinformation represented by equation 2 below to determine command torquesTm1_H1 and Tm2_Hi for the motors MG1 and MG2 in the Hi mode. In thepresent embodiment, the second torque balance information is an equationfor balancing the torques of the power-transmission device 24.Ts1_Hi=Te_ref*r_frTc1_Hi=Ts1_Hi*(−1)*((Zr1/Zs1)+1)Tr2_Hi=To_ref*(Zod/Zo)Ts2_Hi=Tr2_Hi*(Zs2/Zr2)Tcp1_Hi=Tc1_Hi+Ts2_HiTm1_Hi=Tcp1_Hi*(−1)*(Zp1/Zp1d)Tr1_Hi=Ts1_Hi*(Zr1/Zs1)Tc2_Hi=Tr2_Hi*(−1)*((Zs2/Zr2)+1)Tcp2_Hi=Tr1_Hi+Tc2_HiTm2_Hi=Tcp2_Hi*(−1)*(Zp2/Zp2d)  (Equation 2)

The contents of the parameters in each torque balance information aredepicted in Table 1 below.

TABLE 1 Te_ref Target input torque To_ref Target output torque r_frSpeed reduction ratio of the FR switch mechanism 65 (The FR switchmechanism 65 outputs the engine rotation speed to decelerate to 1/r_fr.When the FR switch mechanism 65 is in the forward movement state, r_fris a negative value. When the FR switch mechanism 65 is in the reversemovement state, r_fr is a positive value.) Zs1 Number of teeth of thesun gear S1 in the first planetary gear mechanism 68. Zr1 Number ofteeth of the ring gear R1 in the first planetary gear mechanism 68. Zp1Number of teeth in the first carrier gear Gc1 Zp1d Number of teeth ofthe first motor gear Gm1 Zs2 Number of teeth of the sun gear S2 in thesecond planetary gear mechanism 69. Zr2 Number of teeth of the ring gearR2 in the second planetary gear mechanism 69. Zp2 Number of teeth of thefirst ring outer periphery gear Gr1 Zp2d Number of teeth of the secondmotor gear Gm2 Zo Number of teeth of the second ring outer peripherygear Gr2 Zod Number of teeth of the output gear 71

Next, a method for determining the target input torque Te_ref and thetarget output torque To_ref will be explained. While the target inputtorque Te_ref and the target output torque To_ref can be set optionally,in the present exemplary embodiment the target input torque Te_ref andthe target output torque To_ref are determined so that predeterminedvehicle speed—tractive force characteristics can be achieved in whichthe tractive force changes continuously in response to the vehiclespeed.

FIG. 7 illustrates processing for determining the target output torqueTo_ref. As illustrated in FIG. 7, the control unit 27 has a transmissionrequirement determination unit 84. The transmission requirementdetermination unit 84 determines a required tractive force Tout on thebasis of an accelerator operating amount Aac and an output rotationspeed Nout. The accelerator operating amount Aac is detected by theaccelerator operation detecting unit 51 b. The output rotation speedNout is detected by the vehicle speed detecting unit 37.

The transmission requirement determination unit 84 determines therequired tractive force Tout from the output rotation speed Nout on thebasis of required tractive force characteristics information D1 storedin the storage unit 56. The target-output-torque determination unit 82determines the target output torque To_ref on the basis of the requiredtractive force Tout. Specifically, the target-output-torquedetermination unit 82 determines the target output torque To_ref bymultiplying the required tractive force Tout by a transmission outputratio Rtm. The transmission output ratio Rtm is explained below.

The required tractive force characteristics information D1 is dataindicating the required tractive force characteristics for defining therelationship between the output rotation speed Nout and the requiredtractive force Tout. The required tractive force characteristicscorrespond to the above mentioned predetermined vehicle speed—tractiveforce characteristics. That is, the target output torque To_ref isdetermined so that the tractive force outputted from thepower-transmission device 24 follows the required tractive forcecharacteristics defined by the required tractive force characteristicsinformation D1.

Specifically as illustrated in FIG. 8, the storage unit 56 stores dataLout1 (referred to below as “basic tractive force characteristicsLout1”) indicating basic required tractive force characteristics. Thebasic tractive force characteristics Lout1 are required tractive forcecharacteristics when the accelerator operating amount Aac is at themaximum value, that is, at 100%. The basic tractive forcecharacteristics Lout1 are determined in response to a speed rangeselected by the speed change operating member 53 a. The transmissionrequirement determination unit 84 determines current required tractiveforce characteristics Lout2 by multiplying the basic tractive forcecharacteristics Lout1 by a tractive force ratio FWR and a vehicle speedratio VR.

The storage unit 56 stores tractive force ratio information D2 andvehicle speed ratio information D3. The tractive force ratio informationD2 defines the tractive force ratio FWR with respect to the acceleratoroperating amount Aac. The vehicle speed ratio information D3 defines thevehicle speed ratio VR with respect to the accelerator operating amountAac. The transmission requirement determination unit 84 determines thetractive force ratio FWR and the vehicle speed ratio VR in response tothe accelerator operating amount Aac. The transmission requirementdetermination unit 84 determines the current required tractive forcecharacteristics Lout2 in response to the accelerator operating amountAac by multiplying the basic tractive force characteristics Lout1 by thetractive force ratio FWR in the vertical axis direction which indicatesthe required tractive force and the vehicle speed ratio VR in thehorizontal axis direction which indicates the output rotation speedNout.

The tractive force ratio information D2 defines the tractive force ratioFWR that increases in correspondence to an increase in the acceleratoroperating amount Aac. The vehicle speed ratio information D3 defines thevehicle speed ratio VR which increases in correspondence to an increasein the accelerator operating amount Aac. However, the tractive forceratio FWR is greater than zero when the accelerator operating amount iszero. Similarly, the vehicle speed ratio VR is greater than zero whenthe accelerator operating amount Aac is zero. As a result, the requiredtractive force Tout is a value greater than zero when the acceleratoroperating member 51 a is not being operated. That is, tractive force isbeing outputted from the power-transmission device 24 even when theaccelerator operating member 51 a is not being operated. As a result, abehavior similar to creep generated in a torque converter-type speedchange device is materialized in the EMT-type power-transmission device24.

The required tractive force characteristics information D1 defines therequired tractive force Tout that increases in response to a reductionin the output rotation speed Nout. When the abovementioned speed changeoperating member 53 a is operated, the transmission requirementdetermination unit 84 changes the required tractive forcecharacteristics in response to the speed range selected by the speedchange operating member 53 a. For example, when a down-shift isconducted using the speed change operating member 53 a, the requiredtractive force characteristics information changes from Lout2 to Lout2′as illustrated in FIG. 8. As a result, the upper limit of the outputrotation speed Nout is reduced. That is, the upper limit of the vehiclespeed is reduced.

The required tractive force characteristics information D1 defines thenegative required tractive force Tout with respect to the outputrotation speed Nout that is no less than the predetermined speed. As aresult, the required tractive force Tout is determined to be a negativevalue when the output rotation speed Nout is larger than the upper limitof the output rotation speed in the selected speed range. A brakingforce is generated when the required tractive force Tout is a negativevalue. As a result, a behavior similar to engine brake generated in atorque converter-type speed change device is materialized in theEMT-type power-transmission device 24.

FIG. 9 illustrates processing for determining the target input torqueTe_ref. The target-input-torque determination unit 81 determines thetarget input torque Te_ref on the basis of a transmission requiredhorsepower Htm and an energy management required horsepower Hem.Specifically, the target-input-torque determination unit 81 calculates atransmission required input horsepower Htm_in by adding the energymanagement required horsepower Hem to a value derived by multiplying thetransmission required horsepower Htm by the transmission output ratioRtm. The transmission required horsepower Htm is a horsepower requiredby the power-transmission device 24 for realizing the above mentionedrequired tractive force characteristics, and is calculated bymultiplying the current output rotation speed Nout by the abovementioned required tractive force Tout (see FIG. 8). The energymanagement required horsepower Hem is a horsepower required by thepower-transmission device 24 for charging the below mentioned capacitor64. Therefore, the transmission required input horsepower Htm_in is ahorsepower required for outputting a desired tractive force from thepower-transmission device 24 and for charging the capacitor 64 with thepower-transmission device 24. However, the Hem being a negative valuesignifies that discharging of the capacitor 64 is required.

The target-input-torque determination unit 81 then converts thetransmission required input horsepower Htm_in to a torque and determinesthe target input torque Te_ref so as not to exceed a predetermined upperlimit target input torque Max_Te. Specifically, the target-input-torquedetermination unit 81 calculates a transmission required input torqueTin by dividing the transmission required input horsepower Htm_in by acurrent engine rotation speed Ne. The target-input-torque determinationunit 81 then determines the smaller of a transmission required inputtorque Tin and the upper limit target input torque Max_Te as the targetinput torque Te_ref.

FIG. 10 illustrates processing to determine the upper limit target inputtorque Max_Te. As illustrated in FIG. 10, the upper limit target inputtorque Max_Te is defined by an upper limit target input torque lineLmax_Te+Tpto. Specifically the target-input-torque determination unit 81determines an upper limit target input torque Max_Te+Tpto from the upperlimit target input torque line Lmax_Te+Tpto and the current enginerotation speed Ne.

The upper limit target input torque line Lmax_Te+Tpto is stored in thestorage unit 56 and defines the relationship between the upper limittarget input torque Max_Te+Tpto and the engine rotation speed Ne. Whilethe upper limit target input torque line Lmax_Te+Tpto can be setoptionally, the upper limit target input torque line Lmax_Te+Tpto in thepresent exemplary embodiment is defined so that the upper limit targetinput torque Max_Te+Tpto becomes smaller than a target output torque Tenof the engine 21 determined from the transmission required inputhorsepower Htm_in and the current engine rotation speed Ne.

The upper limit target input torque Max_Te+Tpto derived from the upperlimit target input torque line Lmax_Te+Tpto defines the upper limit ofthe target input torque in which the transmission required input torqueTin as well as a work implement load torque Tpto are combined. The workimplement load torque Tpto is a torque to be distributed to thehydraulic pump through the PTO 22 as described below. Therefore, thetarget-input-torque determination unit 81 calculates the upper limittarget input torque Max_Te as the upper limit of the target input torqueTe_ref by subtracting the work implement load torque Tpto from the upperlimit target input torque Max_Te+Tpto which is derived from the upperlimit target input torque line Lmax_Te+Tpto.

Next, a method for determining the energy management required horsepowerHem will be explained. As illustrated in FIG. 9, the control unit 27 hasan energy management requirement determination unit 85. The energymanagement requirement determination unit 85 determines an energymanagement required horsepower Hem on the basis of a remaining amount ofelectrical power in the capacitor 64.

Specifically, the storage unit 56 stores target capacitor capacityinformation D4. The target capacitor capacity information D4 defines therelationship between the output rotation speed Nout and a targetcapacitor capacity Cp_target. Specifically, the energy managementrequirement determination unit 85 defines the target capacitor capacityCp_target that grows smaller as the output rotation speed Noutincreases. The energy management requirement determination unit 85refers to the target capacitor capacity information D4 to determine thetarget capacitor capacity Cp_target from the output rotation speed Nout.The energy management requirement determination unit 85 determines acurrent capacitor capacity Cp_current from a voltage Vca of thecapacitor 64. The energy management requirement determination unit 85then determines the energy management required horsepower Hem from thefollowing equation 3.Hem=(Cp_target−Cp_current)*P_gain  (Equation 3)

P_gain is a predetermined coefficient. The energy management requirementdetermination unit 85 increases the energy management requiredhorsepower Hem as the current capacitor capacity Cp_current becomessmaller. The energy management requirement determination unit 85increases the energy management required horsepower Hem as the targetcapacitor capacity Cp_target becomes larger.

The control of the engine 21 by the control unit 27 is described indetail below. As described above, the control unit 27 controls theengine by transmitting command signals to the fuel injection device 28.A method for determining the command throttle values for the fuelinjection device 28 will be explained below.

The command throttle value Th_cm is determined on the basis of an enginerequired horsepower Hdm required by the engine 21 (see FIG. 12). Asdescribed above, a portion of the driving power from the engine 21 isdistributed to the power-transmission device 24 and the hydraulic pumps.As a result, the control unit 27 determines the engine requiredhorsepower on the basis of a work implement required horsepower Hptowhich is the horsepower distributed to the hydraulic pumps in additionto the above mentioned transmission required horsepower Htm and theenergy management required horsepower Hem.

As illustrated in FIG. 11, the control unit 27 has a work implementrequirement determination unit 86. The work implement requirementdetermination unit 86 determines the work implement required horsepowerHpto on the basis of a work implement pump pressure Pwp and an operationamount Awo (referred to below as “work implement operation amount Awo”)of the work implement operating member 52 a. In the present exemplaryembodiment, the work implement required horsepower Hpto is a horsepowerdistributed to the work implement pump 23. However, the work implementrequired horsepower Hpto may include a horsepower distributed to thesteering pump 30 and/or the transmission pump 29 as described below.

Specifically, the work implement requirement determination unit 86determines a required flow rate Qdm of the work implement pump 23 fromthe work implement operation amount Awo on the basis of required flowrate information D5. The required flow rate information D5 is stored inthe storage unit 56 and defines the relationship between the requiredflow rate Qdm and the work implement operation amount Awo. The workimplement requirement determination unit 86 determines the workimplement required horsepower Hpto from the required flow rate Qdm andthe work implement pump pressure Pwp. Specifically, the work implementrequirement determination unit 86 determines the work implement requiredhorsepower Hpto using the following equation 4.Hpto=Qdm/ηv*Pwp/ηt  (Equation 4)

ηv is a volume efficiency. ηt is a torque efficiency. The volumeefficiency ηv and the torque efficiency ηt are fixed values determinedin accordance with the characteristics of the work implement pump 23.The work implement pump pressure Pwp is detected by the work implementpump pressure detecting unit 32.

The work implement requirement determination unit 86 determines theabove mentioned work implement load torque Tpto on the basis of the workimplement pump pressure Pwp and a work implement output flow rate Qwo.Specifically, the work implement requirement determination unit 86determines the work implement load torque Tpto using the followingequation 5.Tpto=Qwp*Pwp/ηt  (Equation 5)

Qwp is the displacement volume of the work implement pump. The workimplement pump displacement volume Qwp is calculated from the tilt angledetected by the first tilt angle detecting part 33.

The work implement requirement determination unit 86 determines the workimplement output flow rate Qwo on the basis of the work implementoperation amount Awo. Specifically, the work implement requirementdetermination unit 86 determines the work implement output flow rate Qwoby multiplying the required flow rate Qdm by a work implement outputratio Rpto. The work implement output ratio Rpto is described below. Thecontrol unit 27 controls the discharge capacity of the work implementpump 23 in response to the work implement output flow rate Qwodetermined as described above.

As illustrated in FIG. 12, the control unit 27 has an engine requirementdetermination unit 87. The engine requirement determination unit 87determines the engine required horsepower Hdm on the basis of the workimplement required horsepower Hpto, the transmission required horsepowerHtm, and the energy management required horsepower Hem. Specifically,the engine requirement determination unit 87 determines the enginerequired horsepower Hdm by adding the work implement required horsepowerHpto, the transmission required horsepower Htm, and the energymanagement required horsepower Hem.

As illustrated in FIG. 13, the control unit 27 has a required throttledetermination unit 89. The required throttle determination unit 89determines a command throttle value Th_cm from the engine requiredhorsepower Hdm and the accelerator operating amount Aac.

Specifically, the storage unit 56 stores an engine torque line Let and amatching line Lma. The engine torque line Let defines a relationshipbetween the output torque of the engine 21 and the engine rotation speedNe. The engine torque line Let includes a regulation region La and afull load region Lb. The regulation region La changes in response to thecommand throttle value Th_cm (see La′ in FIG. 13). The full load regionLb includes a rated point Pr and a maximum torque point Pm located onthe low engine rotation speed side from the rated point Pr.

The matching line Lma is information for determining a first requiredthrottle value Th_tm1 from the engine required horsepower Hdm. While thematching line Lma can be set optionally, the matching line Lma in thepresent exemplary embodiment is set so as to pass through a positioncloser to the maximum torque point Pm than the rated point Pr in thefull load region Lb of the engine torque line Let.

The required throttle determination unit 89 determines the firstrequired throttle value Th_tm1 so that the engine torque line Let andthe matching line Lma coincide at a matching point Pma1 where the outputtorque of the engine 21 becomes the torque corresponding to the enginerequired horsepower Hdm. That is, the intersection of an equivalenthorsepower line Lhdm corresponding to the engine required horsepower Hdmand the matching line Lma is set as a first matching point Pma1, and therequired throttle determination unit 89 determines the first requiredthrottle value Th_tm1 so that the regulation region (see “La”) of theengine torque line Let passes through the first matching point Pma1.

The required throttle determination unit 89 determines the lowest of thefirst required throttle value Th_tm1 and a second required throttlevalue Th_ac corresponding to the accelerator operating amount Aac as thecommand throttle value Th_cm.

A method for determining the above mentioned transmission output ratioRtm and the work implement output ratio Rpto will be explained next. Asillustrated in FIG. 12, the control unit 27 has a distribution ratiodetermination unit 88. The distribution ratio determination unit 88determines the transmission output ratio Rtm and the work implementoutput ratio Rpto on the basis of the work implement required horsepowerHpto, the transmission required horsepower Htm, and the energymanagement required horsepower Hem. The output horsepower from theengine 21 is distributed to the work implement pump 23 and thepower-transmission device 24 by the PTO 22. The output horsepower forthe power-transmission device 24 is distributed among the horsepower forthe tractive force of the power-transmission device 24 and thehorsepower for the charging to the capacitor 64. However, when the sumof the work implement required horsepower Hpto, the transmissionrequired horsepower Htm, and the energy management required horsepowerHem becomes greater than the output horsepower from the engine 21, theoutput horsepower from the engine 21 cannot be distributed as each ofthe required values. As a result, the sum of each of the required valuesis limited so as not to exceed the output horsepower from the engine 21by multiplying the work implement required horsepower Hpto and thetransmission required horsepower Htm by the respective output ratiosRpto and Rtm.

Specifically, when the sum of the work implement required horsepowerHpto, the transmission required horsepower Htm, and the energymanagement required horsepower Hem is equal to or less than apredetermined load upper limit horsepower Hmax, the transmission outputratio Rtm and the work implement output ratio Rpto are each set to “1.”That is, the output horsepower of the engine 21 is distributed accordingto each of the required values of the work implement required horsepowerHpto, the transmission required horsepower Htm, and the energymanagement required horsepower Hem. The predetermined load upper limithorsepower Hmax is determined on the basis of the current enginerotation speed Ne. Specifically, the predetermined load upper limithorsepower Hmax is determined from the above mentioned upper limittarget input torque Max_Te+Tpto and the current engine rotation speed Neas illustrated in FIG. 10.

The distribution ratio determination unit 88 sets a value less than 1 asthe transmission output ratio Rtm when the total of the work implementrequired horsepower Hpto, the transmission required horsepower Htm, andthe energy management required horsepower Hem is larger than thepredetermined load upper limit horsepower Hmax. In this case, thedistribution ratio determination unit 88 prioritizes the energymanagement required horsepower Hem and determines the transmissionoutput ratio Rtm and the work implement output ratio Rpto. That is, thedistribution ratio determination unit 88 divides the work implementrequired horsepower Hpto and the transmission required horsepower Htminto priority portions and ratio portions. The distribution ratiodetermination unit 88 preferentially distributes the output horsepowerfrom the engine 21 as in the following order.

-   1. Energy management required horsepower Hem-   2. Priority portion Hpto_A of the work implement required horsepower    Hpto-   3. Priority portion Htm_A of the transmission required horsepower    Htm-   4. Ratio portion Hpto_B of the work implement required horsepower    Hpto-   5. Ratio portion Htm_B of the transmission required horsepower Htm

For example, as illustrated in FIG. 14, while the sum from the energymanagement required horsepower Hem to the ratio portion Hpto_B of thework implement required horsepower Hpto (Hem_A+Hpto_A+Htm_A+Hpto_B) isless than the predetermined load upper limit horsepower Hmax, when thesum from the energy management required horsepower Hem_A to the ratioportion Htm_B of the transmission required horsepower Htm is greaterthan the predetermined load upper limit horsepower Hmax, the ratioportion of the transmission required horsepower Htm is corrected fromHtm_B to Htm_B′ which is less than Htm_B so that the sum becomes equalto or less than the predetermined load upper limit horsepower Hmax. Theratio of the corrected transmission required horsepower Htm′ withrespect to the uncorrected transmission required horsepower Htm isdetermined as the transmission output ratio Rtm.

Controls during a V-shaped work as an example of controlling during workof the work vehicle 1 according to the present exemplary embodiment willbe explained next. FIG. 15 illustrates operations of the work vehicle 1during a V-shaped work. A V-shaped work typically involves loading aload such as earth from a pile 300 in which a conveyance substance suchas earth is accumulated onto the bed of a dump truck 200. As illustratedin FIG. 15, the V-shaped work involves five work aspects which are (1)moving forward to approach the pile 300, (2) plunging into the pile 300to load the load into the bucket 12 (referred to as “digging” below),(3) moving in reverse to move away from the pile 300, (4) moving forwardto approach the dump truck 200 (referred to below as “dump approach”)and lowering the load from the bucket 12 onto the bed of the dump truck200 (referred to as “dumping” below), and (5) moving in reverse to moveaway from the dump truck 200.

FIG. 16 is a timing chart illustrating changes in each parameter of thework vehicle 1 carrying out the V-shaped work. FIG. 16A illustrates thevehicle speed. The chain double-dashed line in FIG. 16A indicates thevehicle speed of a conventional torque converter-type work vehicle as acomparative example. FIG. 16B illustrates the engine rotation speed. Thechain double-dashed line in FIG. 16B indicates the engine rotation speedof a conventional torque converter-type work vehicle as a comparativeexample. FIG. 16C illustrates the output torque of the engine 21. Thechain double-dashed line in FIG. 16C indicates the output torque of aconventional torque converter-type work vehicle as a comparativeexample. The solid line in FIG. 16D indicates the work implement pumppressure. The dashed line in FIG. 16D indicates the displacement volumeof the work implement pump 23. That is, FIG. 16D illustrates the load onthe work implement pump 23. The solid line in FIG. 16E indicates theboom operation amount. The dashed line in FIG. 16E indicates the bucketoperation amount. A positive operation amount in FIG. 16E signifies anoperation of raising the work implement 3, and a negative operationamount signifies an operation of lowering the work implement 3. Thesolid line in FIG. 16F indicates the accelerator operation amount. Theaccelerator operating amount is maximum except for a portions when thework aspect is being switched during the V-shaped work in the presentembodiment. The dashed line in FIG. 16F indicates the command throttlevalue. FIG. 16G illustrates the speed ranges selected by the speedchange operating member 53 a. FIG. 16H illustrates the selectionpositions (FNR positions) of the FR operating member 54 a.

As described above, the work vehicle moves forward to approach the pile300 in the forward movement work aspect (1). As a result, an operationin which the load on the work implement 3 increases is not basicallybeing conducted as illustrated in FIG. 16E, and the load on the workimplement pump 23 is small as illustrated in FIG. 16D. Therefore, thehorsepower of the engine 21 is mainly distributed to thepower-transmission device 24.

As illustrated in FIG. 16A, the work vehicle 1 begins to move from astopped state and then accelerates in the forward movement work aspect(1). As illustrated in FIG. 8, the transmission requirementdetermination unit 84 in the work vehicle 1 according to the presentexemplary embodiment determines the required tractive force Tout on thebasis of the output rotation speed Nout and the accelerator operatingamount Aac, and determines the transmission required horsepower Htm bymultiplying the output rotation speed Nout by the transmission requiredhorsepower Htm. Therefore, the transmission required horsepower Htm issmall because the vehicle speed when beginning to move is low. However,because the tractive force is generated even before the engine rotationspeed increases, the acceleration when beginning to move is superior incomparison to the conventional torque converter-type vehicle asillustrated in FIG. 16A. The energy required for acceleration can beadditionally supplied from the electrical power of the capacitor 64.

Next when the vehicle speed rises and the transmission requiredhorsepower Htm increases, the engine required horsepower Hdm increases.As a result, the command throttle value Th_cm increases as illustratedin FIG. 16F due to the matching point Pma1 illustrated in FIG. 13 movingalong the matching line Lma. Consequently, the engine rotation speedincreases as illustrated in FIG. 16B. However in the conventional torqueconverter-type work vehicle, the engine rotation speed rises due to theengine load growing smaller when there is no load on the work implement.As a result, the consumption amount of the fuel increases. Conversely inthe work vehicle 1 of the present exemplary embodiment, the enginerotation speed is kept low because the command throttle value Th_cm isdetermined on the basis of the transmission required horsepower Htm.Accordingly, fuel consumption can be improved.

Next in the digging work aspect (2), the work vehicle 1 plunges into thepile 300 to load the load into the bucket. As illustrated in FIG. 16A,the vehicle speed is low in the digging work aspect. As a result, theoutput torque from the power-transmission device 24 is large and therequired horsepower in the power-transmission device 24 is small.However, operation of the work implement 3 is conducted in the diggingwork aspect (2) as illustrated in FIG. 16E. As a result, the horsepowerof the engine 21 is distributed to the work implement pump 23.Therefore, the distribution of the horsepower to the power-transmissiondevice 24 and the work implement pump 23 is important in the diggingwork aspect (2) because there may be a shortage of digging power if thehorsepower of the engine 21 is too small.

As illustrated in FIG. 16G, the operator shifts down the speed rangefrom the second speed to the first speed by first operating the speedchange operating member 53 a when starting the digging. When the speedrange is shifted down to the first speed in the work vehicle 1 accordingto the present exemplary embodiment, the basic tractive forcecharacteristics are changed from second speed characteristics to firstspeed characteristics. As a result, the required tractive forcecharacteristics are changed from Lout2 to Lout2′ as illustrated in FIG.8 for example. In this case, the required tractive force Tout isdetermined to be a negative value when the vehicle speed (outputrotation speed Nout) is greater than the upper limit in the first speed.As a result, braking force is generated.

When the bucket 12 plunges into the pile 300, the vehicle speeddecreases. In this case, the transmission requirement determination unit84 determines the required tractive force Tout and the transmissionrequired horsepower Htm in response to the vehicle speed according tothe required tractive force characteristics for the first speed.

During digging, the operator conducts an operation to raise the boom 11.In this case, the work implement required horsepower Hpto illustrated inFIG. 11 increases and as a result the engine required horsepower Hdmillustrated in FIG. 12 increases. As a result, the command throttlevalue Th_cm increases due to the matching point Pma1 illustrated in FIG.13 moving along the matching line Lma. Consequently, the engine rotationspeed increases.

During digging, the operator may intermittently perform the operation ofraising the bucket 12 as illustrated in FIG. 16E. In this case, the workimplement required horsepower Hpto illustrated in FIG. 11 repeatedlyincreases and decreases and as a result the engine required horsepowerHdm illustrated in FIG. 12 repeatedly increases and decreases. Thecommand throttle value Th_cm changes as illustrated in FIG. 16F due tothe matching point Pma1 illustrated in FIG. 13 moving along the matchingline Lma in response to the changes in the engine required horsepowerHdm, and as a result the engine rotation speed is adjusted asillustrated in FIG. 16B.

In the work aspect (3) of moving in reverse and moving away from thepile 300, the work vehicle 1 moves in reverse to move away from the pile300. As a result, an operation in which the load on the work implement 3increases is basically not being conducted as illustrated in FIG. 16E,and the load on the work implement pump 23 is small as illustrated inFIG. 16D.

During reverse travel, the work vehicle 1 first accelerates in the sameway as in during forward travel. As a result, the vehicle speed towardthe rear increases. As illustrated in FIG. 16A, the acceleration of thework vehicle 1 according to the present exemplary embodiment whenbeginning to move is superior in comparison to the conventional torqueconverter-type vehicle even when beginning to move in reverse in thesame way as when beginning to move forward.

The work vehicle 1 conducts the next deceleration when moving in reverseand moving away from the earth. During deceleration, the transmissionrequired horsepower Htm decreases because the required tractive forceTout decreases. As a result, the command throttle value Th_cm decreasesas illustrated in FIG. 16F due to the matching point Pma1 illustrated inFIG. 13 moving along the matching line Lma. Consequently, the enginerotation speed decreases as illustrated in FIG. 16B. Duringdeceleration, the braking force may be required because the requiredtractive force Tout becomes a negative value. For example as illustratedin FIG. 16Q immediately after the FR operating member 54 a duringreverse movement of the work vehicle 1 is switched from the reversemovement position to the forward movement position, the FR operatingmember 54 a is set to the forward movement position, but the brakingforce is required when the work vehicle 1 is moving in reverse. When thebraking force is generated, kinetic energy absorbed as braking force isregenerated in the engine 21 or in the capacitor 64 through thepower-transmission device 24. When energy is regenerated in the engine21, fuel consumption can be improved. Moreover, when energy isregenerated in the capacitor 64, the timing for using the energy can beadjusted.

In the dump approach/dumping work aspect, the work vehicle 1 next movesforward and approaches the dump truck 200 and lowers the load from thebucket 12 onto the bed of the dump truck 200. The work vehicle 1accelerates while carrying the load in the bucket 12 during the dumpapproach. At this time, the load on the work implement pump 23 is largedue to the operation of raising the bucket 12 being conducted. When thevehicle speed is too fast in the dump approach/dumping work aspect, thework vehicle 1 may reach the dump truck 200 before the bucket 12 issufficiently raised. As a result, the proper operability of the vehiclespeed and the work implement 3 is important.

The load on the work implement pump 23 is large during the dumpapproach. As a result, the engine rotation speed is increased to handlethe high load of the work pump in the conventional torque converter-typework vehicle. In this case, the absorption torque of the torqueconverter also increases and as a result the vehicle speed increases. Asa result, there is a need to adjust the vehicle speed by using anoperating member such as the brake, an inching pedal, or a cut-offpedal. Conversely, the work implement required horsepower Hpto isdetermined in response to the operation of the work implement operationdetecting unit 52 b in the work vehicle 1 according to the presentexemplary embodiment. As a result, the necessary power can be suppliedto the work implement pump 23 through the operation of the workimplement operation detecting unit 52 b. Moreover, the vehicle speed canbe adjusted easily by operating the accelerator operating member 51 abecause the target output torque To_ref is determined on the basis ofthe accelerator operating amount Aac. As a result, the vehicle speed canbe adjusted easily and the work implement 3 can be operated easilywithout complicated driving operations.

The engine output of the work vehicle 1 according to the presentexemplary embodiment is adjusted on the basis of horsepower. As aresult, the engine can be controlled in a high engine efficiency regionwith low rotation and high torque. As illustrated in FIG. 10, thetarget-input-torque determination unit 81 determines the target inputtorque Te_ref so as not to exceed the predetermined upper limit targetinput torque Max_Te. Therefore, the target input torque Te_ref isdetermined so that excess torque for increasing the engine rotationspeed remains. As a result, a reduction in engine rotation speed can besuppressed even if the load on the engine 21 is large.

During dumping, the operator completes the raising of the boom 11, andlowers the bucket 12 while reducing the accelerator operating amount.Therefore, the required tractive force Tpto is reduced and thetransmission required horsepower Htm is reduced in accordance with areduction in the accelerator operating amount. The work implementrequired horsepower Hpto is reduced due to the completion of the raisingof the boom 11. As a result, the command throttle value Th_cm decreasesas illustrated in FIG. 16(F) due to the matching point Pma1 illustratedin FIG. 13 moving along the matching line Lma. Consequently, the enginerotation speed decreases as illustrated in FIG. 16(B).

The controls for the work aspect (5) of moving in reverse and movingaway from the dump truck 200 are the same as those of work aspect (3)for moving in reverse and moving away from the pile 300, and thus anexplanation thereof will be omitted.

The work vehicle 1 according to the present exemplary embodiment has thefollowing features. The control unit 27 is able to achieve the desiredinput torque to the power-transmission device 24 and the desired outputtorque from the power-transmission device 24 by determining the commandtorques Tm1_ref and Tm2_ref to the motors MG1 and MG2 from the balanceof the torques of the power-transmission device 24. As a result,predetermined tractive force characteristics can be obtained accurately.Generally, a work vehicle is required to conduct work while the tractiveforce and the loads on the work implement fluctuate greatly. Therefore,the ability to adjust the input torque and the output torque to thepower-transmission device to desired values is desirable to achieve abalance between the driving power and the operation of the workimplement. By adjusting the target input torque Te_ref and the targetoutput torque To_ref in the work vehicle 1 according to the presentexemplary embodiment, the desired input torque to the power-transmissiondevice 24 and the desired output torque from the power-transmissiondevice 24 can be achieved. As a result, a work vehicle combining bothoperability and drivability can be realized.

The transmission requirement determination unit 84 determines therequired tractive force Tout on the basis of the accelerator operatingamount Aac and the output rotation speed Nout. Therefore, the requiredtractive force Tout is determined on the basis of the acceleratoroperating amount Aac in addition to the output rotation speed Nout. Thetarget output torque To_ref is determined on the basis of theaccelerator operating amount Aac because the target-output-torquedetermination unit 82 determines the target output torque To_ref on thebasis of the required tractive force Tout. As a result, the operationalfeeling of the operator can be improved.

The transmission requirement determination unit 84 determines therequired tractive force Tout from the output rotation speed Nout on thebasis of the required tractive force characteristics information D1. Thetransmission requirement determination unit 84 determines the requiredtractive force characteristics information D1 on the basis of theaccelerator operating amount Aac. As a result, the required tractiveforce Tout can be determined on the basis of the accelerator operatingamount Aac by determining the required tractive force characteristicsinformation D1 in response to the accelerator operating amount Aac.

The transmission requirement determination unit 84 determines thecurrent required tractive force characteristics Lout2 by multiplying thebasic tractive force characteristics Lout1 by the tractive force ratioFWR and the vehicle speed ratio VR. The transmission requirementdetermination unit 84 determines the tractive force ratio FWR and thevehicle speed ratio VR in response to the accelerator operating amountAac. As a result, the required tractive force characteristics Lout2 canbe determined in response to the accelerator operating amount Aac byusing the tractive force ratio FWR and the vehicle speed ratio VR inresponse to the accelerator operating amount Aac.

The required tractive force characteristics information D1 defines therequired tractive force Tout as a negative value with respect to theoutput rotation speed Nout that is no less than the predetermined speed.As a result, the required tractive force Tout becomes a negative valuewhen the output rotation speed Nout is not less than the predeterminedspeed. That is, when the output rotation speed Nout is high, thepower-transmission device 24 is controlled so as to generate a brakingforce. For example, when the required tractive force characteristicschange from Lout2 to Lout2′ in a state corresponding to the point P onthe required tractive force characteristics in FIG. 8, the requiredtractive force Tout is changed from a positive value to a negativevalue. As a result, braking force is generated. Therefore, a behaviorsimilar to engine brake generated due to down-shifting occurring in atorque converter-type speed change device is materialized in theEMT-type power-transmission device 24.

The target-input-torque determination unit 81 determines the targetinput torque Te_ref on the basis of the transmission required horsepowerHtm and the energy management required horsepower Hem. As a result, thetarget input torque Te_ref for the power-transmission device 24 can bedetermined so that a horsepower required for outputting a tractive forcecorresponding to the required tractive force from the power-transmissiondevice 24, and a required horsepower for storing electrical power in thecapacitor 64.

The target-input-torque determination unit 81 determines the upper limitof the target input torque Te_ref from the upper limit target inputtorque line Lmax_Te+Tpto and the engine rotation speed Ne. As a result,a value less than target output torque of the engine 21 determined fromthe engine required horsepower Hdm and the engine rotation speed Nebecomes the upper limit of the target input torque Te_ref. Therefore,the target input torque Te_ref is determined so that excess torque forincreasing the engine rotation speed Ne remains. As a result, a decreasein the engine rotation speed Ne due to overloading can be suppressed.

The distribution ratio determination unit 88 sets a value less than oneas the transmission output ratio Rtm when the total of the workimplement required horsepower Hpto, the transmission required horsepowerHtm, and the energy management required horsepower Hem is larger thanthe predetermined load upper limit horsepower Hmax. As a result, whilethe value of the transmission required horsepower Htm is reduced whendetermining the target input torque Te_ref when the total of the workimplement required horsepower Hpto, the transmission required horsepowerHtm, and the energy management required horsepower Hem is larger thanthe predetermined load upper limit horsepower Hmax, the value of theenergy management required horsepower Hem is maintained. That is, theenergy management required horsepower Hem is prioritized more than thetransmission required horsepower Htm when determining the target inputtorque Te_ref. Consequently, the energy management required horsepowerHem is prioritized and the output horsepower from the engine 21 can bedistributed, and as a result predetermined electrical power can beensured in the capacitor 64.

The matching line Lma is set so as to pass through a position closer tothe maximum torque point Pm than the rated point Pr in the full loadregion Lb of the engine torque line Let. As a result, the enginerotation speed Ne at the matching point Pma1 is smaller in comparison towhen the matching line Lma is set so as to pass through a positioncloser to the rated point Pr than the maximum torque point Pm in thefull load region Lb of the engine torque line Let. As a result, the fuelconsumption can be improved.

The engine requirement determination unit 87 determines the enginerequired horsepower Hdm on the basis of the work implement requiredhorsepower Hpto, the transmission required horsepower Htm, and theenergy management required horsepower Hem. As a result, the enginerequired horsepower Hdm can be determined which is suited to the driveof the work implement 3 and the drive of the travel device 25corresponding to the operations of the operator and is suited tocharging the capacitor 64.

The above mentioned power-transmission device 24 has the first planetarygear mechanism 68 and the second planetary gear mechanism 69. However,the number of the planetary gear mechanisms provided in thepower-transmission device is not limited to two. The power-transmissiondevice may only have one planetary gear mechanism. Alternatively, thepower-transmission device may have three or more planetary gearmechanisms. FIG. 17 is a schematic view of a configuration of apower-transmission device 124 provided in a work vehicle according to asecond exemplary embodiment. Other configurations of the work vehicleaccording to the second exemplary embodiment are the same as those ofthe work vehicle 1 according to the above exemplary embodiment and thusexplanations thereof are omitted. The same reference numerals areprovided in FIG. 17 for the configurations which are the same as thepower-transmission device 24 according to the above exemplaryembodiment.

As illustrated in FIG. 17, the power-transmission device 124 has a speedchange mechanism 166. The speed change mechanism 166 has a planetarygear mechanism 168, a first transmission shaft 167, a secondtransmission shaft 191, and a second transmission shaft gear 192. Thefirst transmission shaft 167 is coupled to the FR switch mechanism 65.The planetary gear mechanism 168 and the second transmission shaft gear192 are disposed on the same shaft as the first transmission shaft 167and the second transmission shaft 191.

The planetary gear mechanism 168 has a sun gear S1, a plurality ofplanet gears P1, a carrier C1 that supports the plurality of planetgears P1, and a ring gear R1. The sun gear S1 is coupled to the firsttransmission shaft 167. The plurality of planet gears P1 mesh with thesun gear S1 and are supported in a rotatable manner by the carrier C1.The carrier C1 is fixed to the second transmission shaft 191. The ringgear R1 meshes with the plurality of planet gears P1 and is able torotate. A ring outer periphery gear Gr1 is provided on the outerperiphery of the ring gear R1. The second motor gear Gm2 is fixed to theoutput shaft 194 of the second motor MG2 and the second motor gear Gm2meshes with the ring outer periphery gear Gr1.

The second transmission shaft gear 192 is coupled to the secondtransmission shaft 191. The second transmission shaft gear 192 mesheswith the output gear 71, and the rotation of the second transmissionshaft gear 192 is outputted to the output shaft 63 via the output gear71.

The speed change mechanism 166 has a first high-speed gear (referred tobelow as “first H-gear GH1”), a second high-speed gear (referred tobelow as “second H-gear GH2”), a first low-speed gear (referred to belowas “first L-gear GL1”), a second low-speed gear (referred to below as“second L-gear GL2”), a third transmission shaft 193, and a Hi/Loswitching mechanism 170.

The first H-gear GH1 and the first L-gear GL1 are disposed on the sameshaft as the first transmission shaft 167 and the second transmissionshaft 191. The first H-gear GH1 is coupled to the first transmissionshaft 167. The first L-gear GL1 is coupled to the second transmissionshaft 191. The second H-gear GH2 meshes with the first H-gear GH1. Thesecond L-gear GL2 meshes with the first L-gear GL1. The second H-gearGH2 and the second L-gear GL2 are disposed on the same shaft as thethird transmission shaft 193, and are disposed so as to be able torotate in relation to the third transmission shaft 193. The thirdtransmission shaft 193 is coupled to the output shaft of the first motorMG1.

The Hi/Lo switch mechanism 170 is a mechanism for switching thedrive-power transmission path of the power-transmission device 124between a high-speed mode (Hi mode) in which the vehicle speed is highand a low-speed mode (Lo mode) in which the vehicle speed is low. TheHi/Lo switch mechanism 170 has a H-clutch CH that is ON during the Himode and a L-clutch CL that is ON during the Lo mode. The H-clutch CHconnects and disconnects the second H-gear GH2 and the thirdtransmission shaft 193. The L-clutch CL connects and disconnects thesecond L-gear GL2 and the third transmission shaft 193.

Next the operations of the power-transmission device 124 according tothe second exemplary embodiment will be explained. FIG. 18 illustratesthe rotation speeds of each of the motors MG1 and MG2 with respect tothe vehicle speed of the work vehicle according to the second exemplaryembodiment. In FIG. 18, the solid line represents the rotation speed ofthe first motor MG1, and the dashed line represents the rotation speedof the second motor MG2. In an A region (Lo mode) with a vehicle speedfrom zero to V1, the L-clutch CL is ON (connected) and the H-clutch CHis OFF (disconnected). Because the H-clutch CH is OFF in the A region,the second H-gear GH2 and the third transmission shaft 193 aredisconnected. Because the L-clutch CL is ON, the second L-gear GL2 andthe third transmission shaft 193 are connected.

In the A region, the driving power from the engine 21 is inputted to thesun gear S1 via the first transmission shaft 167, and the driving poweris outputted from the carrier C1 to the second transmission shaft 191.Conversely, the driving power inputted to the sun gear S1 is transmittedfrom the planet gear P1 to the ring gear R1 and outputted through thering outer periphery gear Gr1 and the second motor gear Gm2 to thesecond motor MG2. The second motor MG2 functions mainly as a generatorin the A region, and a portion of the electrical power generated by thesecond motor MG2 is stored in the capacitor 64.

The first motor MG1 functions mainly as an electric motor in the Aregion. The driving power of the first motor MG1 is outputted to thesecond transmission shaft 191 along a path from the third transmissionshaft 193 to the second L-gear GL2 to the first L-gear GL1. The drivingpower joined by the second transmission shaft 191 in this way istransmitted through the second transmission shaft gear 192 and theoutput gear 71 to the output shaft 63.

In a B region (Hi mode) in which the vehicle speed exceeds V1, theH-clutch CH is ON (connected) and the L-clutch CL is OFF (disconnected).Because the H-clutch CH is ON in the B region, the second H-gear GH2 andthe third transmission shaft 193 are connected. Because the L-clutch CLis OFF, the second L-gear GL2 and the third transmission shaft 193 aredisconnected.

In the B region, the driving power from the engine 21 is inputted to thesun gear S1 and the driving power is outputted from the carrier C1 tothe second transmission shaft 191. The driving power from the engine 21is outputted from the first H-gear GH1 through the second H-gear GH2 andthe third transmission shaft 193 to the first motor MG1. The first motorMG1 functions mainly as a generator in the B region, and thus a portionof the electrical power generated by the first motor MG1 is stored inthe capacitor 64.

The driving power of the second motor MG2 is outputted to the secondtransmission shaft 191 along a path from the second motor gear Gm2 tothe ring outer periphery gear Gr1 to the ring gear R1 to the carrier C1.The driving power joined by the second transmission shaft 191 in thisway is transmitted through the second transmission shaft gear 192 andthe output gear 71 to the output shaft 63.

The control of the power-transmission device 124 in the work vehicleaccording to the second exemplary embodiment is the same as the controlof the power-transmission device 24 in the above exemplary embodiment.However, the structure of the power-transmission device 124 is differentfrom that of the power-transmission device 24, and the torque balanceinformation is different from the above information. Specifically, afirst torque balance information in the second exemplary embodiment isrepresented by the following equation 6.Ts1_Low=Te_ref*r_frTc1_Low=Ts1_Low*(−1)*((Zr1/Zs1)+1)Tr1_Low=Ts1_Low*(Zr1/Zs1)Tcm1_Low=To_ref*(−1)*(Zod/Zo)+Tc1_LowTm1_Low=Tcm1_Low*(−1)*(Zm1_Low/Zm1d_Low)Tm2_Low=Tr1_Low*(−1)*(Zm2/Zm2d)  (Equation 6)

A second torque balance information in the second exemplary embodimentis represented by the following equation 7.Tc1_Hi=To_ref*(−1)*(Zod/Zo)Tr1_Hi=Tc1_Hi*(−1)*(1/(Zs/Zr+1))Ts1_Hi=Tr1_Hi*(Zs/Zr)Tsm1_Hi=Ts1+Te_ref*r_frTm1_Hi=Tsm1_Hi*(−1)*(Zm1_Hi/Zm1d_Hi)Tm2_Hi=Tr1_Hi*(−1)*(Zm2/Zm2d)  (Equation 7)

The contents of the parameters in each of the types of torque balanceinformation are depicted in Table 2 below.

TABLE 2 Te_ref Target input torque To_ref Target output torque r_frSpeed reduction ratio of the FR switch mechanism 65 (The FR switchmechanism 65 outputs the engine rotation speed to decelerate to 1/r_fr.When the FR switch mechanism 65 is in the forward movement state, r_fris a negative value. When the FR switch mechanism 65 is in the reversemovement state, r_fr is a positive value) Zs1 Number of teeth of the sungear S1 in the planetary gear mechanism 168. Zr1 Number of teeth of thering gear R1 in the planetary gear mechanism 168. Zm1d_Hi Number ofteeth of the first H-gear GH1 Zm1d_Low Number of teeth of the firstL-gear GL1 Zm1_Hi Number of teeth of the second H-gear GH2 Zm1_LowNumber of teeth of the second L-gear GL2 Zm2 Number of teeth of the ringouter periphery gear Gr1 Zm2d Number of teeth of the second motor gearGm2 Zo Number of teeth of the second transmission shaft gear 192 ZodNumber of teeth of the output gear 71

The present invention is not limited to the above exemplary embodimentsand various changes and modifications may be made without departing fromthe spirit of the invention.

The present invention is not limited to the above mentioned wheel loaderand may be applied to another type of work vehicle such as a bulldozer,a tractor, a forklift, or a motor grader.

The present invention may be applicable to another type of speed changedevice such as a HMT without being limited to the EMT. In this case, thefirst motor MG1 functions as a hydraulic motor and a hydraulic pump. Thesecond motor MG2 functions as a hydraulic motor and a hydraulic pump.The first motor MG1 and the second motor MG2 are variable capacitorpump/motors, and the capacities are controlled by the control unit 27controlling the tilt angle of the skew plate or the inclined shaft. Thecapacities of the first motor MG1 and the second motor MG2 arecontrolled so that the command torques Tm1_ref and Tm2_ref calculated inthe same way as in the above exemplary embodiments are outputted.

The configuration of the power-transmission device 124 is not limited tothe configuration of the above exemplary embodiment. For example, thecoupling and disposition of each of the elements of the two planetarygear mechanisms 68 and 69 are not limited to the coupling anddisposition of the above exemplary embodiments. The configuration of thepower-transmission device 124 is not limited to the configuration of theabove exemplary embodiment. For example, the coupling and disposition ofeach of the elements of the planetary gear mechanism 168 are not limitedto the coupling and disposition of the above exemplary embodiment.

The torque balance information is not limited to the equations forbalancing the torque as in the above exemplary embodiments. For example,the torque balance information may be in the format of a table or a map.The torque balance information is not limited to the above mentioned twotypes of torque balance information of the first torque balanceinformation and the second torque balance information. Three or moretypes of torque balance information may be used corresponding to aselectable number of modes in the power-transmission device 124.

The speed change operating member 53 a may have a kick-down switch. Akick-down switch is an operating member for lowering the speed range ofthe power-transmission device 124 by one step or a plurality of stepsfrom the current speed range. The operator is able to lower the speedrange of the power-transmission device 124 from the current speed rangeto a low speed range by operating the kick-down switch.

The determination of the work implement required horsepower Hpto and thework implement load torque Tpto in the above embodiments takes intoconsideration the required horsepower and the load torque of the workimplement pump 23 for the work implement 3, but may also take intoconsideration the required horsepower and the load torque of a hydraulicpump for auxiliary equipment. The hydraulic pump for auxiliary equipmentmay include the abovementioned transmission pump 29. That is, the workimplement required horsepower Hpto and the work implement load torqueTpto may be determined while considering the required horsepower and theload torque of the abovementioned transmission pump 29 in addition tothose of the work implement pump 23.

Alternatively, the hydraulic pump for the auxiliary equipment mayinclude the above mentioned steering pump 30. That is, the workimplement required horsepower Hpto and the work implement load torqueTpto may be determined while considering the required horsepower and theload torque of the abovementioned steering pump 30 in addition to thoseof the work implement pump 23.

Alternatively, when the work vehicle 1 is provided with a cooling fanfor cooling the engine 21, a fan motor for driving the cooling fan, anda fan pump for driving the fan motor, the hydraulic pump for auxiliaryequipment may be the fan pump. That is, the work implement requiredhorsepower Hpto and the work implement load torque Tpto may bedetermined while additionally considering the required horsepower andthe load torque of the fan pump.

Alternatively, the work implement required horsepower Hpto and the workimplement load torque Tpto may be determined while considering a portionor all of the required horsepower and the load torque of the abovementioned hydraulic pumps in addition to those of the work implementpump 23.

The priority levels for the distribution of the output horsepower fromthe engine 21 by the distribution ratio determination unit 88 are notlimited to the levels of the above exemplary embodiments and may bemodified. While the priority level of the ratio portion Htm_B of thetransmission required horsepower Htm is lower than the priority level ofthe ratio portion Hpto_B of the work implement required horsepower Hptoin the above exemplary embodiments, the priority levels of the ratioportion Htm_B of the transmission required horsepower Htm and the ratioportion Hpto_B of the work implement required horsepower Hpto may be thesame.

For example as illustrated in FIG. 19, when the sum of the workimplement required horsepower Hpto, the transmission required horsepowerHtm, and the energy management required horsepower Hem is greater thanthe predetermined load upper limit horsepower Hmax, the ratio portionHtm_B of the transmission required horsepower Htm and the ratio portionHpto_B of the work implement required horsepower Hpto may be multipliedby the same ratio α (<1) so that the sum becomes equal to or less thanthe load upper limit horsepower Hmax whereby the ratio portion Htm_B andthe ratio portion Hpto_B are corrected respectively to Htm_B′ andHpto_B′. That is, Htm_B′ equals Htm_B*α and Hpto_B′ equals Hpto_B*α. Theratio of the corrected transmission required horsepower Htm′ withrespect to the uncorrected transmission required horsepower Htm isdetermined as the transmission output ratio Rtm. The ratio of thecorrected work implement required horsepower Hpto′ with respect to theuncorrected work implement required horsepower Hpto is determined as thework implement output ratio Rpto.

INDUSTRIAL APPLICABILITY

The present invention demonstrates the effect of allowing a high degreeof freedom in setting the tractive force characteristics and obtainingpredetermined tractive force characteristics with accuracy in the workvehicle. Therefore the present invention is useful as a work vehicle anda control method for the work vehicle.

What is claimed is:
 1. A work vehicle, comprising: an engine; ahydraulic pump driven by the engine; a work implement driven byhydraulic fluid discharged from the hydraulic pump; a travel devicedriven by the engine; a power-transmission device configured to transmitdriving power from the engine to the travel device; and a control unitfor controlling the power-transmission device; the power-transmissiondevice including an input shaft; an output shaft; a gear mechanism fortransmitting rotation of the input shaft to the output shaft, the gearmechanism including a plurality of planetary gear mechanisms and amode-switching mechanism for selectively switching a drive-powertransmission path in the power-transmission device among a plurality ofmodes; and a motor connected to a rotating element of the plurality ofplanetary gear mechanisms; and, the power-transmission device beingconfigured so that a rotation speed ratio of the output shaft withrespect to the input shaft is changed by changing a rotation speed ofthe motor, and the control unit including a target-input-torquedetermination unit that determines a target input torque which is atorque target value inputted to the power-transmission device; atarget-output-torque determination unit that determines a target outputtorque which is a torque target value outputted from thepower-transmission device; a storage unit for storing torque balanceinformation for defining a relationship between the target input torqueand the target output torque to achieve a balance of the torques of thepower-transmission device; and a command-torque determination unit thatuses the torque balance information to determine a command torque forthe motor from the target input torque and the target output torque. 2.The work vehicle according to claim 1, further comprising a vehiclespeed detecting unit for detecting a vehicle speed; an acceleratoroperating member; an accelerator operation detecting unit for detectingan operation amount of the accelerator operating member; the controlunit further having a transmission requirement determination unit fordetermining a required tractive force on the basis of the vehicle speedand the operation amount of the accelerator operating member; and thetarget-output-torque determination unit determining the target outputtorque on the basis of the required tractive force.
 3. The work vehicleaccording to claim 2, wherein the transmission requirement determinationunit determines the required tractive force from the vehicle speed onthe basis of required tractive force characteristics that define arelationship between the vehicle speed and the required tractive force;and the transmission requirement determination unit determines therequired tractive force characteristics in response to the operationamount of the accelerator operating member.
 4. The work vehicleaccording to claim 3, wherein the transmission requirement determinationunit determines the required tractive force characteristics bymultiplying the basic required tractive force characteristics by atractive force ratio and a vehicle speed ratio; and the transmissionrequirement determination unit determines the tractive force ratio andthe vehicle speed ratio in response to the operation amount of theaccelerator operating member.
 5. The work vehicle according to claim 4,further comprising a speed change operating member; the transmissionrequirement determination unit selecting the basic required tractiveforce characteristics in response to an operation of the speed changeoperating member.
 6. The work vehicle according to claim 4, whereintractive force ratio information and vehicle speed ratio information arestored in the storage unit; the tractive force ratio informationdefining the tractive force ratio with respect to the operation amountof the accelerator operating member; and the vehicle speed ratioinformation defining the vehicle speed ratio with respect to theoperation amount of the accelerator opening member.
 7. The work vehicleaccording to claim 2, wherein the required tractive forcecharacteristics define the required tractive force that is a negativevalue with respect to the vehicle speed equal to or above apredetermined speed.
 8. The work vehicle according to claim 2, furthercomprising an energy reservoir unit for storing energy generated in themotor; the control unit further has an energy management requirementdetermination unit for determining an energy management requiredhorsepower on the basis of a remaining amount of energy in the energyreservoir unit; the transmission requirement determination unitdetermining a transmission required horsepower on the basis of thevehicle speed and the operation amount of the accelerator operatingmember; and the target-input-torque determination unit determining thetarget input torque on the basis of the transmission required horsepowerand the energy management required horsepower.
 9. The work vehicleaccording to claim 8, further comprising a work implement operatingmember for operating the work implement; the control unit furtherincluding a work implement requirement determination unit fordetermining a work implement required horsepower on the basis of theoperation amount of the work implement operating member; and an enginerequirement determination unit for determining an engine requiredhorsepower on the basis of the work implement required horsepower, thetransmission required horsepower, and the energy management requiredhorsepower; the target-input-torque determination unit determining anupper limit of the target input torque from an upper limit target inputtorque line and the engine rotation speed; and the upper limit targetinput torque line defining, as the upper limit of the target inputtorque, a value that is less than the target output torque of the enginedetermined from the engine required horsepower and the engine rotationspeed.
 10. The work vehicle according to claim 9, wherein the controlunit further has a distribution ratio determination unit for determininga transmission output ratio; and the distribution ratio determinationunit sets a value less than one as the transmission output ratio whenthe total of the work implement required horsepower, the transmissionrequired horsepower, and the energy management required horsepower islarger than a predetermined load upper limit horsepower; and thetarget-input-torque determination unit determines the target inputtorque on the basis of the energy management required horsepower and avalue derived by multiplying the transmission required horsepower by thetransmission output ratio.
 11. The work vehicle according to claim 10,wherein the target-output-torque determination unit determines thetarget output torque on the basis of a value derived by multiplying therequired tractive force by the transmission output ratio.
 12. The workvehicle according to claim 1, wherein the control unit further includesan engine requirement determination unit for determining an enginerequired horsepower; and a required throttle determination unit fordetermining a required throttle value, the storage unit stores an enginetorque line that defines a relationship between the output torque of theengine and the engine rotation speed, and a matching line fordetermining the required throttle value from the engine requiredhorsepower; the engine torque line includes a regulation region and afull load region; the regulation region changes in response to therequired throttle value; the full load region includes a rated point anda maximum torque point located on the low engine rotation speed sidefrom the rated point; the required throttle determination unitdetermines the required throttle value so that the engine torque lineand the matching line coincide at a matching point where the outputtorque of the engine becomes the torque corresponding to the enginerequired horsepower; and the matching line is set to pass through aposition closer to the maximum torque point than the rated point in thefull load region of the engine torque line.
 13. The work vehicleaccording to claim 12, further comprising a work implement operatingmember for operating the work implement; a vehicle speed detecting unitthat detects a vehicle speed; an accelerator operating member; anaccelerator operation detecting unit for detecting an operation amountof the accelerator operating member; and an energy reservoir unit forstoring energy generated in the motor; the control unit furtherincluding a work implement requirement determination unit fordetermining a work implement required horsepower on the basis of theoperation amount of the work implement operating member; a transmissionrequirement determination unit for determining a transmission requiredhorsepower on the basis of the vehicle speed and the operation amount ofthe accelerator operating member; and an energy management requirementdetermination unit for determining an energy management requiredhorsepower on the basis of a remaining amount of energy in the energyreservoir unit; the engine requirement determination unit determiningthe engine required horsepower on the basis of the work implementrequired horsepower, the transmission required horsepower, and theenergy management required horsepower.
 14. The work vehicle according toclaim 1, wherein the plurality of modes includes a first mode and asecond mode; and the command-torque determination unit determines thecommand torque for the motor from a first torque balance information inthe first mode, and determines the command torque for the motor from asecond torque balance information in the second mode.
 15. A method forcontrolling a work vehicle, the work vehicle comprising: apower-transmission device having an input shaft; an output shaft; a gearmechanism for transmitting rotation of the input shaft to the outputshaft, the gear mechanism including a plurality of planetary gearmechanisms and a mode-switching mechanism for selectively switching adrive-power transmission path in the power-transmission device between aplurality of modes; and a motor connected to a rotating element of theplurality of planetary gear mechanisms, the power-transmission deviceconfigured to change a rotation speed ratio of the output shaft withrespect to the input shaft by changing the rotation speed of the motor,the method comprising: a step for determining a target input torque thatis a torque target value inputted to the power-transmission device; astep for determining a target output torque that is a torque targetvalue outputted from the power-transmission device; and a step for usingtorque balance information for defining a relationship between thetarget input torque and the target output torque so that a balance ofthe torques in the power-transmission device is achieved, to determine acommand torque for the motor from the target input torque and the targetoutput torque.